Materials and methods for the preparation of nanocomposites

ABSTRACT

Disclosed herein is an isolable colloidal particle comprising a nanoparticle and an inorganic capping agent bound to the surface of the nanoparticle, a method for making the same in a biphasic solvent mixture, and the formation of structures and solids from the isolable colloidal particle. The process can yield photovoltaic cells, piezoelectric crystals, thermoelectric layers, optoelectronic layers, light emitting diodes, ferroelectric layers, thin film transistors, floating gate memory devices, phase change layers, and sensor devices.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 14/117,070,filed Jul. 25, 2014, which is the U.S. national phase of InternationalApplication No. PCT/US12/38218 filed May 6, 2012, which claims thepriority benefit under 35 U.S.C. § 119(e) of U.S. ProvisionalApplication No. 61/486,595, filed May 16, 2011, the entire disclosuresof which are incorporated herein by reference.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with governmental support under grants from theU.S. National Science Foundation (DMR-0847535; IIP-10471801) and U.S.Department of Energy (DE-AC02-06CH11357). The U.S. government hascertain rights in the invention.

FIELD OF DISCLOSURE

The disclosure generally relates to materials and methods for thepreparation of nanocomposites. More specifically, the disclosure relatesto inorganic capped colloidal materials and the methods of depositingthese inorganic capped colloidal materials on a substrate to formnanocomposites. Still more specifically, the disclosure relates to theselective deposition and formation of nanocomposites on a substrate.

All-inorganic colloidal nanocrystals (NC) were synthesized by replacingorganic capping ligands on chemically synthesized nanocrystals withmetal-free inorganic ions such as S²⁻, HS⁻, Se²⁻, HSe⁻, HSe⁻, Te²⁻,HTe⁻, TeS₃ ²⁻, OH⁻ and NH₂ ⁻. These simple ligands adhered to the NCsurface and provided colloidal stability in polar solvents. The use ofsmall inorganic ligands instead of traditional ligands with longhydrocarbon tails facilitated the charge transport between individualnanocrystals and opened up interesting opportunities for deviceintegration of colloidal nanostructures.

BRIEF DESCRIPTION OF RELATED TECHNOLOGY

Nanoscale materials, materials with at least one dimension between about1 and 1000 nm, have increasingly garnered attention due to theirpotential electronic, photophysical, chemical, and medicinal effects.The large-scale industrial application of nanoscale materials hasgenerally focused on the formation of nanometer thick films and/ornanometer wide wires. Many of these industrially-applied nanoscalematerials display extraordinary electronic and photophysical properties,but more often the materials lack the features that originally drewscientific interest toward nanocrystals, nanorods, and nanowires.

Attempts to incorporate the physical properties of nanocrystals,nanorods, and nanowires into films or bulk solids have led to theself-assembly of ordered nanoarrays. See Shevchenko et al, “Structuraldiversity in binary nanoparticle superlattices” in Nature, 439, 55-59(2006). These self-assembled ordered nanoarrays have been produced fromstable colloidal solutions of nanomaterials. For example, close-packednanocrystal films have been made by spin-coating or drop casting ofcolloidal solutions. Often these films show short range ordering, butforces such as entropy, electrostatics, and van der Waals interactionscan cause these materials to self-assemble into superlattices. Thesetechniques have afforded binary superlattices with tunable electronicstructures based on the colloidal materials employed in the synthesis.

Though some single-component and binary superlattices exhibit desirablephysical and electronic properties, these materials are not robustenough for large scale advanced material applications and theirsynthesis is not general enough to provide easy production of idealizedmaterials.

A larger-scale approach to the synthesis of solid state materialsencompassing nanocrystals is the impregnation and forced crystallizationof nanocrystals from melts of inorganic materials. See Sootsman et. al,“Large Enhancements in the Thermoelectric Power Factor of Bulk PbTe atHigh Temperature by Synergistic Nanostructuring” in Angew. Chem. Int.Ed. 47, 8618-22 (2008). This rapid quenching approach can providenanocrystalline material in bulk inorganic phases but lacks anymethodology for the formation of ordered nanoarrays in the bulkmaterial.

While the synthesis of solid state materials with ordered arrays ofnanoscale materials has progressed to the point where nanocrystals canbe deposited in ordered arrays on a surface, the use of these orderedarrays are hampered by the insulating ligands generally used in themanufacture of the nanocrystal. The practical use of these nanocrystalshas been discovered through the blending of these organic solublenanocrystals with polymers. See for example U.S. Pat. No. 7,457,508. Forexample, nanocomposites of nanocrystals and conjugated polymers canyield tunable semiconducting photonic structures, and with uniqueoptical, electrical, magnetic, electrochromic, and chemical properties.See for example U.S. Pat. No. 7,200,318.

The majority of applications wherein these advanced materials will beapplicable employ inorganic solids as the functional material. Oneexample of an applicable inorganic solid that incorporates nanoscalematerials is the fabrication of inorganic nanocomposites described inU.S. patent application Ser. No. 11/330,291. This methodology involvesthe codeposition of a nanocrystalline material with an inorganic matrixprecursor from a homogeneous hydrazine solution, a technique similar tothe deposition of nanocrystalline materials in polymers as described inJ. W. Lee et al., Advanced Materials 2000, 12, 1102. This methodologyfails to provide the selectivity of structure for the synthesis oftunable semiconducting materials, does not prevent the carboncontamination of the synthesized inorganic nanocomposite, and requires ahighly toxic, hypergolic solvent. Hence, the industrial applicability ofthe methodology is limited by material requirements, and theoverwhelming health and safety concerns.

Generally, the prior art does not sufficiently teach or suggest theisolation of colloidal particles useful for the easy and safe depositionof a pure colloidal material. Moreover, the prior art does not teach orsuggest a methodology based on isolable colloidal nanomaterials for theproduction of superlattices, inorganic matrices, hetero-alloys, alloys,and the like.

SUMMARY OF INVENTION

Disclosed herein is a composition and a method for making thatcomposition having a nanoparticle capped with an inorganic cappingagent. The method generally includes at least two immiscible solventsand the exchange of an organic capping agent on a nanoparticle with theherein described inorganic capping agent.

Another aspect of the disclosure is a composition made of thenanoparticle and the inorganic capping agent. The composition isisolable, can be purified, and importantly may display approximately thesame opto-electronic characteristics as the nanoparticle with an organiccapping agent.

Yet another aspect of the disclosure is the deposition of thecomposition on a substrate. Herein, the composition can be deposited asthin or bulk films by a variety of techniques with short or long rangeordering of the nanoparticles. The deposited composition, importantly,displays approximately the same opto-electronic characteristics as thecomposition in solution.

Still another aspect of the disclosure is the thermal decomposition ofthe deposited composition to form inorganic matrices with imbeddednanoparticles. The annealed composition has an inorganic matrix thatcorresponds to the thermal decomposition product of the inorganiccapping agent. Additionally, as the annealed composition can be producedfrom the deposited composition with ordered nanoparticles (arrays), theannealed composition can have ordered arrays of nanoparticles in a solidstate matrix. The annealed composition can also, importantly, displayapproximately the same optical characteristics as the depositedcomposition.

Additionally, the deposited composition can be thermally treated suchthat the composition partially or wholly anneals. The formed alloy canhave discrete regions with elemental compositions that approximate thenanoparticle and the solid state matrix as made through the abovereferenced thermal decomposition or the alloy can be annealed to asingle phase.

The herein disclosed materials and methods provide a route to new anduseful solid state materials that can exhibit for examplethermoelectric, piezoelectric, ferroelectric, phase change andelectroluminescent characteristics. These solid state materials can beused in devices like photovoltaic cells, piezoelectric crystals,thermoelectric layers, optoelectronic layers, light emitting diodes,ferroelectric layers, thin film transistors, floating gate memorydevices, phase change layers, and sensor devices. Uses of and methods ofassembling such devices are generally described in U.S. Ser. No.12/142,454, U.S. Pat. No. 7,348,224, U.S. Ser. No. 12/328,788, U.S. Ser.No. 11/865,325, U.S. Ser. No. 11/864,877, PCT/US2007/018015, U.S. Ser.No. 12/285,158, U.S. Ser. No. 12/108,500, U.S. Ser. No. 11/942,441,PCT/US2008/002246, U.S. Ser. No. 11/584,304, U.S. Ser. No. 12/241,333,U.S. Ser. No. 12/179,807, U.S. Ser. No. 12/155,015, PCT/US2006/022250,U.S. Pat. No. 7,517,702, U.S. Ser. No. 12/050,676, U.S. Ser. No.12/130,075, U.S. Ser. No. 11/789,344, PCT/KR2007/002885, U.S. Ser. No.11/704,623, U.S. Ser. No. 11/856,086, U.S. Ser. No. 11/604,746,PCT/US2008/000271, U.S. Pat. No. 7,485,526, U.S. Ser. No. 12/079,088,U.S. Ser. No. 12/032,252, PCT/US2008/005430, U.S. Ser. No. 12/050,284,and U.S. Ser. No. 11/803,261 all of which are incorporated herein byreference. Solid state materials in accordance with the descriptionsherein may be used in assembling any of these and similar devices.

Additional features of the invention may become apparent to thoseskilled in the art from a review of the following detailed description,taken in conjunction with the drawings, the examples, and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference should bemade to the following detailed description and accompanying drawingswherein:

FIG. 1A illustrates a red colored colloidal dispersion of CdSe NCsundergoes the phase transfer from toluene to FA upon exchange of theoriginal organic surface ligands with S²⁻;

FIG. 1B illustrates Absorption and PL spectra of 5.5 nm CdSe NCs cappedwith organic ligands and S²⁻ ligands dispersed in toluene and FArespectively;

FIG. 1C illustrates FTIR spectra of 5.5 nm CdSe NCs with differentcombinations of ligands and solvents;

FIG. 1D illustrates NC size-distributions measured by Dynamic LightScattering for ˜12 nm CdSe NCs capped with organic ligands and S²⁻ ions.Inset shows TEM image of the NCs capped with S²⁻ ligand;

FIG. 1E illustrates TEM images of S²⁻-DDA⁺ capped CdS nanorods;

FIG. 1F illustrates Absorption spectra of 5.5 nm CdSe NCs capped withdifferent ligands;

FIG. 2A illustrates Absorption and PL spectra of CdSe/ZnS core/shell NCscapped with S²⁻ ligands dispersed in FA; inset shows the luminescence ofNC dispersion upon illumination with 365 nm UV light;

FIG. 2B illustrates Absorption spectra of InP NCs capped with organicligands and S²⁻ ligands dispersed in toluene and FA respectively. Insetshows the optical photograph of colloidal dispersion of InP NCs in FA;

FIG. 2C illustrates Absorption spectra of Au NCs capped withdodecanethiol and S²⁻ ligands dispersed in toluene and FA, respectively.Inset shows the optical photograph of a colloidal dispersion of Au NCsin FA;

FIG. 2D illustrates DLS size-distribution plots for Au NCs capped withthe organic and S²⁻ ligands;

FIG. 3A illustrates ESI-MS spectrum of (NH₄)₂TeS₃ solution, in which theinset compares an experimental high-resolution mass-spectrum withsimulated spectra for TeS₃H⁻ and TeS₃ ⁻ ions;

FIG. 3B illustrates Absorption spectra for (NH₄)₂TeS₃, for CdSe NCscapped with the organic ligands in toluene and for CdSe NCs capped withTeS₃ ²⁻ ions in formamide;

FIG. 4A illustrates Absorption and PL spectra of OH⁻ and NH₂ ⁻ cappedCdSe NCs dispersed FA, in which the inset shows a photograph of thecolloidal solution;

FIG. 4B illustrates Absorption spectra of ZnSe NCs capped witholeylamine and OH⁻ ligands dispersed in toluene and FA, respectively;

FIG. 5A illustrates removal of the organic ligands from the surface ofCdSe NCs by HBF₄ treatment, and for simplicity, monodentate binding modeof carboxylate group is shown, and the proposed mechanism can hold forother chelating and bridging modes as well;

FIG. 5B illustrates Absorption and PL spectra of ligand-free colloidalCdSe and CdSe/ZnS NCs obtained after HBF₄ treatment;

FIG. 5C illustrates FTIR spectra of organically capped and ligand-freeCdSe NCs after HBF₄ treatment;

FIG. 6A is a plot of drain current I_(D) vs. drain-source voltageV_(DS), measured as different gate voltages V_(G) for the field-effecttransistors (FETs) assembled from colloidal NCs capped with (NH₄)₂S forCdSe;

FIG. 6B is a plot of drain current I_(D) vs. drain-source voltageV_(DS), measured as different gate voltages V_(G) for the field-effecttransistors (FETs) assembled from colloidal NCs capped with (NH₄)₂S forCdSe/CdS core-shells;

FIG. 6C is a plot of drain current I_(D) vs. drain-source voltageV_(DS), measured as different gate voltages V_(G) for the field-effecttransistors (FETs) assembled from colloidal NCs capped with (NH₄)₂S forCdSe/ZnS core-shells, showing insets of the photoluminescence spectrum(left) and a photograph (right) for CdSe/ZnS NC film capped with (NH₄)₂Sligands annealed at 250° C. for 30 min;

FIG. 6D is a plot of I_(D) vs. V_(G) at constant V_(DS)=2V used tocalculate current modulation and linear regime field-effect mobility forFETs using CdSe;

FIG. 6E is a plot of I_(D) vs. V_(G) at constant V_(DS)=2V used tocalculate current modulation and linear regime field-effect mobility forCdSe/CdS NCs;

FIG. 6F is a plot of I_(D) vs. V_(G) measured at constant V_(DS)=4V in aFET assembled from CdSe/ZnS NCs, where L=80 μm, W=1500 μm, 100 nm SiO₂gate oxide;

FIG. 7 is the photoluminecent spectrum of SH⁻ capped CdSe NCs dispersedin formamide;

FIG. 8 is the FTIR spectra of FA, DMSO, H₂O and an aqueous solution of(NH₄)₂S;

FIG. 9 is the ESI Mass spectra of different sulfide and polysulfide ionsused as ligands for colloidal nanocrystals;

FIG. 10A is the Experimental and simulated mass-spectra showing theregions of S₅Na⁻, where experimental spectra were obtained from redcolored Na₂S_(x) aqueous solution and different spectral regions werecollected from the corresponding ESI-MS spectrum in FIG. 9;

FIG. 10B is the Experimental and simulated mass-spectra showing theregions of S₆Na⁻, where experimental spectra were obtained from redcolored Na₂S_(x) aqueous solution and different spectral regions werecollected from the corresponding ESI-MS spectrum in FIG. 9;

10C is the Experimental and simulated mass-spectra showing the regionsof S₇Na⁻, where experimental spectra were obtained from red coloredNa₂S_(x) aqueous solution and different spectral regions were collectedfrom the corresponding ESI-MS spectrum in FIG. 9;

FIG. 11A is the Experimental and simulated mass spectra showing theregions of S₃ ²⁻/S₃H⁻, where experimental spectra were obtained from theyellow colored aqueous solution (NH₄)₂S and different spectral regionswere collected from the corresponding spectrum in FIG. 9;

FIG. 11B is the Experimental and simulated mass spectra showing theregions of S₂O₃ ²⁻/S₂O₃H⁻, where experimental spectra were obtained fromthe yellow colored aqueous solution (NH₄)₂S and different spectralregions were collected from the corresponding spectrum in FIG. 9;

FIG. 11C is the Experimental and simulated mass spectra showing theregions of S₄ ²⁻/S₄H⁻, where experimental spectra were obtained from theyellow colored aqueous solution (NH₄)₂S and different spectral regionswere collected from the corresponding spectrum in FIG. 9;

FIG. 11D is the Experimental and simulated mass spectra showing theregions of S₅H⁻, where experimental spectra were obtained from theyellow colored aqueous solution (NH₄)₂S and different spectral regionswere collected from the corresponding spectrum in FIG. 9;

FIG. 11E is the Experimental and simulated mass spectra showing theregions of S₆H⁻, where experimental spectra were obtained from theyellow colored aqueous solution (NH₄)₂S and different spectral regionswere collected from the corresponding spectrum in FIG. 9;

FIG. 12 illustrates powder X-ray diffraction patterns of CdSe NCs cappedwith the organic ligands and same NCs capped with TeS₃ ²⁻ ligands afterannealing at 120° C.;

FIG. 13 is XRD patterns of TeS₃ ²⁻ capped PbS NCs;

FIG. 14 is XRD patterns of 5 nm CdSe NCs capped with different ligands;

FIG. 15 is FTIR spectra CdSe NCs capped with organic ligands, KOH andNaNH₂;

FIG. 16 is a table comparing the ζ-potential values for NCs capped withsoft bases (S²⁻ and SH⁻) and hard base (OH⁻), where soft bases exhibitmore negative ζ-potential, indicating higher binding affinity, for NCswith softer acidic cites, and in contrast, the hard base shows higherbinding affinity for NCs with harder acidic sites;

FIG. 17 is UV-visible absorption spectra of ˜5 nm CdSe NCs capped withorganic ligands and same NCs after NOBF₄ treatment, dispersed in hexaneand FA, respectively;

FIG. 18A is a plot of I_(D) vs. V_(G) at constant V_(DS)=30V used tocalculate the saturation field-effect mobility for FETs using CdSe,where L=80 μm, W=1500 μm, 100 nm SiO₂ gate oxide;

FIG. 18B is a plot of of I_(D) vs. V_(G) at constant V_(DS)=30V used tocalculate the saturation field-effect mobility for FETs using CdSe/CdScore-shell NCs, where L=80 μm, W=1500 μm, 100 nm SiO₂ gate oxide;

FIG. 18C is a plot of I_(D) vs. V_(G) at constant V_(DS)=30V used tocalculate the saturation field-effect mobility for FETs using CdSe/ZnScore-shell NCs, where L=80 μm, W=1500 μm, 100 nm SiO₂ gate oxide;

FIG. 19A is a FET assembled from 3.9 nm CdSe NCs capped with TeS₃ ²⁻ligands annealed at 250° C. for 30 min, and is a plot of drain currentI_(D) vs. drain-source voltage V_(DS), measured as different gatevoltages V_(G), where L=80 μm, W=1500 μm, 100 nm SiO₂ gate oxide;

FIG. 19B is a FET assembled from 3.9 nm CdSe NCs capped with TeS₃ ²⁻ligands annealed at 250° C. for 30 min, and is a plot of I_(D) vs. V_(G)at constant V_(DS)=2V used to calculate current modulation and linearregime field-effect mobility, where L=80 μm, W=1500 μm, 100 nm SiO₂ gateoxide;

FIG. 20 is a schematic of top-gate transistor used here; and

FIG. 21A is a plot of the Transfer characteristics of transistors basedon S²⁻ capped CdSe nanocrystals annealed at 200° C.;

FIG. 21B is a plot of the output characteristics of transistors based onS²⁻ capped CdSe nanocrystals annealed at 200° C.;

FIG. 22A shows Dynamic light scattering (DLS) size histogram for Fe₂O₃NCs (15 nm) capped with POMs;

FIG. 22B shows Zeta potential (ζ) measurements for CdSe and Fe₂O₃ NCsbefore and after PO₄ ³⁻ ligand capping; (C);

FIG. 22C shows FTIR spectra of Fe₂O₃ NCs;

FIG. 22D shows CdSe NCs before and after organic ligands removal;

FIG. 22E shows Zeta potential measurements for positively charged CdSeNCs in FA with addition of PO₄ ³⁻ ligands;

FIG. 22F shows Absorption spectra of CdSe NCs capped with differentoxo-ligands;

FIG. 23A shows a TEM image of Fe₂O₃ NCs capped with Na₂MoO₄ in NFA;

FIG. 23B shows a TEM image of Fe₂O₃ NCs capped with Na₂MoO₄ in FA;

FIG. 23C shows a TEM image of Fe₂O₃ NCs capped with Na₂HAsO₄ in NFA;

FIG. 23D shows a TEM image of Fe₂O₃ NCs capped with Na₂HAsO₄ in FA;

FIG. 23E shows a TEM image of Fe₂O₃ NCs capped with Na₃[PMo₁₂O₄₀] inNFA;

FIG. 23F shows a TEM image of Fe₂O₃ NCs capped with Na₃[PMo₁₂O₄₀] in FA;

FIG. 23G shows a TEM image of Fe₂O₃ NCs capped with K₆[P₂Mo₁₈O₆₂] inNFA;

FIG. 23H shows a TEM image of Fe₂O₃ NCs capped with K₆[P₂MO₁₈O₆₂] in FA;

FIG. 24A shows a TEM image of CdSe NCs capped with organic ligands;

FIG. 24B shows a TEM image of CdSe NCs capped with DMF after HBF₄treatment in FA;

FIG. 24C shows a TEM image of CdSe NCs capped with Na₃VO₄ in FA;

FIG. 24D shows a TEM image of CdSe NCs capped with Na₂S₂O₃ in FA;

FIG. 24E shows a TEM image of CdSe NCs capped with Na₂MoS₄ in FA;

FIG. 24F shows a TEM image of CdSe NCs capped with Na₂MoO₄ in FA;

FIG. 24G shows a TEM image of CdSe NCs capped with Na₂HAsO₄ in FA;

FIG. 24H shows a TEM image of CdSe NCs capped with Na₃PO₄ in FA;

FIG. 25A shows Zeta potential titration of CdSe NCs by addition ofdifferent amounts of oxo-ligands in DMF;

FIG. 25B shows Zeta potential titration of Fe₂O₃ NCs by addition ofdifferent amounts of oxo-ligands in DMF;

FIG. 26 shows ζ-potential measurements of Fe₂O₃ NCs solutions in DMFwith different pHs;

FIG. 27A shows the XRay diffraction pattern of growth on zinc blendeCdSe NCs at room temperature;

FIG. 27B shows the XRay diffraction pattern of growth on wurtzite CdSeNCs at room temperature;

FIG. 27C shows the absorption spectra of zinc blende CdSe-xSCd with x=1. . . 10 and TEM images of zinc blende CdSe-10SCd with (NH₄)₂S as aprecursor of sulfide and cadmium oleate, Cd(OA)₂, as a precursor ofcadmium, and the nanocrystals are washed after each monolayer;

FIG. 27D shows the absorption spectra of wurtzite CdSe-xSCd with x=1 . .. 10 and TEM images of wurtzite CdSe-10SCd with (NH₄)₂S as a precursorof sulfide and cadmium oleate, Cd(OA)₂, as a precursor of cadmium, andthe nanocrystals are washed after each monolayer;

FIG. 27E shows the absorption spectra of zinc blende CdSe-xSCd with x=1. . . 10 and TEM images of zinc blende CdSe-10SCd with QDs stabilized byoleylamine either when they are sulfide rich or cadmium rich;

FIG. 27F shows the absorption spectra of wurtzite CdSe-xSCd with x=1 . .. 10 and TEM images of wurtzite CdSe-10SCd with QDs stabilized byoleylamine either when they are sulfide rich or cadmium rich;

FIG. 28A-F shows TEM images and absorption spectra of different NCs withdifferent inorganic ligands;

FIG. 29A shows absorption spectra of CdSe-xSCd for x=1 . . . 10;

FIG. 29B shows a TEM image of CdSe

FIG. 29C shows a TEM image of CdSe-10SCd;

FIG. 30A shows Absorption spectra of CdSe(nanoplatelets absorbing at 462nm)/xSCd, all the spectra have been done at the same concentration;

FIG. 30B shows variation of the first excitonic peak absorptionwavelength, wavelength of photoluminescence maximum and the FWHM infunction of the thickness of the shell;

FIG. 30C is a picture of the aliquots CdSe/SCd;

FIG. 31 shows absorption spectra of CdS-xSZn doped with manganese in thethird layer for x=1 . . . 6 and emission spectrum of CdS-6SZn:Mn showingthe phsophorescence coming from Mn²⁺;

FIG. 32A shows Ligand exchange of TOP capped InAs NCs with(N₂H₄)₂(N₂H₅)₂In₂Se₄ upon phase transfer from hexane to N₂H₄;

FIG. 32B shows Absorption spectra of InAs NCs capped with TOP and(N₂H₄)₂(N₂H₅)₂In₂Se₄ in toluene and N₂H₄, respectively;

FIG. 32C is a TEM image of InAs NCs capped with (N₂H₄)₂(N₂H₅)₂In₂Se₄;

FIG. 32D shows Absorption spectra of InP NCs capped with TOP/TOPO and(N₂H₄)₃(N₂H₅)₄Sn₂S₆ in toluene and N₂H₄, respectively;

FIG. 32E is a TEM image of InP NCs capped with (N₂H₄)₃(N₂H₅)₄Sn₂S₆;

FIG. 32F shows size distribution of InAs NCs measured by Dynamic LightScattering after ligand exchange with (N₂H₄)₂(N₂H₅)₂In₂Se₄;

FIG. 33A shows Fourier transform infrared spectra of InAs NCs cappedwith TOP (red line) and (N₂H₄)₂(N₂H₅)₂In₂Se₄ before (green line) andafter (blue line) annealing at 200° C.;

FIG. 33B shows Fourier transform infrared spectra for InP NCs cappedwith myristic acid (red line) and (N₂H₄)₂(N₂H₅)₂In₂Se₄ ligand before(green line) and after (blue line) annealing at 200° C.;

FIG. 33C shows Powder X-ray (CuK_(α) radiation) diffraction of InAs NCscapped with (N₂H₄)₂(N₂H₅)₂In₂Se₄ before (red line) and after (blue line)annealing at 280° C.;

FIG. 33D shows Powder X-ray (CuK_(α) radiation) diffraction of InP NCscapped with (N₂H₄)₂(N₂H₅)₂In₂Se₄ before (red line) and after (blue line)annealing at 280° C.;

FIG. 33E shows Thermogravimetric scans for (N₂H₄)₂(N₂H₅)₂In₂Se₄ (blackline) and (N₂H₄)₂(N₂H₅)₂In₂Se₄ capped InAs NCs;

FIG. 33F shows Absorption spectra taken with an integrated sphere forthin film of (N₂H₄)₂(N₂H₅)₂In₂Se₄ capped InAs NCs before and afterannealing at 200° C.;

FIG. 34A shows a plot of drain current (I_(D)) vs drain-source voltage(V_(DS)) measured at different gate voltages (V_(G)) for field-effecttransistor (FETs) assembled from InAs NCs capped with Cu₇S₄ ² where L=50mm, W=1500 mm, 100 nm gate oxide;

FIG. 34B shows a plot of drain current (I_(D)) vs drain-source voltage(V_(DS)) measured at different gate voltages (V_(G)) for field-effecttransistor (FETs) assembled from InAs NCs capped with Sn₂S₆ ⁴⁻ whereL=50 mm, W=1500 mm, 100 nm gate oxide;

FIG. 34C shows a plot of drain current (I_(D)) vs drain-source voltage(V_(DS)) measured at different gate voltages (V_(G)) for field-effecttransistor (FETs) assembled from InAs NCs capped with Sn₂Se₆ ⁴⁻ whereL=50 mm, W=1500 mm, 100 nm gate oxide;

FIG. 34D shows a plot of I_(D) vs V_(G) at constant V_(DS)=2V used tocalculate current modulation and linear electron mobility for FETsassembled from InAs NCs capped with Cu₇S₄ ²⁻ where L=50 mm, W=1500 mm,100 nm gate oxide;

FIG. 34E shows a plot of I_(D) vs V_(G) at constant V_(DS)=2V used tocalculate current modulation and linear electron mobility for FETsassembled from InAs NCs capped with Sn₂S₆ ⁴⁻ where L=50 mm, W=1500 mm,100 nm gate oxide;

FIG. 34F shows a plot of I_(D) vs V_(G) at constant V_(DS)=2V used tocalculate current modulation and linear electron mobility for FETsassembled from InAs NCs capped with Sn₂Se₆ ⁴⁻ where L=50 mm, W=1500 mm,100 nm gate oxide;

FIG. 35A shows photoresponse measured at a bias of 2V as a function ofexcitation energy in closed-packed films of InAs NC capped with In₂Se₄²⁻, where L=50 μm, W=1500 μm, 100 nm gate oxide;

FIG. 35B photoresponse measured at a bias of 2V as a function ofexcitation energy in closed-packed films of InAs NC capped with Cu₇S₄²⁻, where L=50 μm, W=1500 μm, 100 nm gate oxide;

FIG. 35C shows photoresponse measured at a bias of 2V as a function ofexcitation energy in closed-packed films of InAs NC capped with Sn₂S₆⁴⁻, where L=50 μm, W=1500 μm, 100 nm gate oxide;

FIG. 35D shows Gate dependent photoresponse for the FETs assembled fromcolloidal InAs NCs capped with Sn₂S₆ ⁴⁻, where L=50 μm, W=1500 μm, 100nm gate oxide;

FIG. 36A shows a plot of drain current (I_(D)) vs drain-source voltage(V_(DS)) measured at different gate voltages (V_(G)) for the FETsassembled from colloidal InP NCs capped with In₂Se₄ ²⁻, where L=4.5 μm,W=7800 μm, 100 nm gate oxide;

FIG. 36B shows a plot of I_(D) vs V_(G) at constant V_(DS)=30 V used tocalculate current modulation and saturation mobility for FETs assembledfrom colloidal InP NCs capped with In₂Se₄ ²⁻, where L=4.5 μm, W=7800 μm,100 nm gate oxide;

FIG. 36C shows I-V curves of films of InP NCs capped with In₂Se₄ ²⁻ as afunction of excitation energy;

FIG. 36D shows Photoresponse measured at the bias of 3 V as function ofexcitation energy in closed-packed films of InP NC capped with In₂Se₄²⁻;

FIG. 37A shows a schematic representation of surface charges for K₂Scapped CdSe NCs, before treatment with strongly binding metal ionsM^(n+);

FIG. 37B shows a schematic representation of surface charges for K₂Scapped CdSe NCs after treatment with strongly binding metal ions M^(n+),where the variations of ζ-potential of K₂S capped CdSe NCs in formamidein presence of 5 mM various metal ions, and the inset showing thevariation of ζ-potential in the Mg²⁺, Ca²⁺, Ba²⁺ series plotted againstthe metal ion radius relative to the ionic radius of Cd²⁺;

FIG. 37C shows UV-visible absorption and PL spectra of CdSe NCs cappedwith K₂S, before and after addition of 5 mM Cd²⁺ ions;

FIG. 37D shows Room temperature PL efficiency of CdSe/S²⁻ NCs in thepresence of different metal ions, plotted against ζ-potential;

FIG. 38A shows Photoluminescence spectra for CdSe/ZnS NCs capped withK₂S in the presence of different concentrations of Cd²⁺ ions;

FIG. 38B shows PL decay profiles for CdSe/ZnS NCs with different surfacemodifications, where the excitation wavelength was 400 nm;

FIG. 39A shows UV-visible absorption spectra for a film of CdSe NCscapped with K₂S before and after treatment with Cd²⁺ ions, where themeasurements were carried out using an integrating sphere;

FIG. 39B shows schematics of NC linking with metal ions;

FIG. 39C shows schematics of a field-effect transistor used for studiesof electronic coupling in the NC solids;

FIG. 40A generally illustrates the effect of metal ion on the electronmobility in all-inorganic NC solids, where output characteristics, I_(D)vs. V_(DS) as a function of gate voltage V_(G) for the field-effectdevices (FETs) using films of 4.2 nm CdSe NCs capped with K₂S(CdSe/S²⁻/K⁺);

FIG. 40B illustrates the effect of metal ion on the electron mobility inall-inorganic NC solids, where output characteristics, I_(D) vs. V_(DS)as a function of gate voltage V_(G) for the field-effect devices (FETs)using films of 4.2 nm CdSe NCs capped with CdSe/S²⁻ NCs treated withCd²⁺ ions (CdSe/S²⁻/Cd²⁺);

FIG. 40C illustrates transfer characteristics, I_(D) vs. V_(G) atV_(DS)=3V for FET devices using CdSe/S²⁻/K⁺ and CdSe/S²⁻/Cd²⁺ NC solids,where the inset shows the turn-on gate voltage (V_(th)) for CdSe/S²⁻ NCstreated with different cations;

FIG. 40D illustrates a plot of output characteristic;

FIG. 40 E illustrates a plot of output characteristic;

FIG. 40F illustrates a plot of transfer characteristics for FETs made ofK₂S capped 4.5 nm InAs NCs before (InAs/S²⁻/K⁺) and after(InAs/S²⁻/In³⁺) treatment with In³⁺ ions;

FIG. 40G illustrates a plot showing the effect of different metal ionsin the field-effect electron mobility in CdSe/S²⁻ NC film;

FIG. 40H illustrates a plot showing the effect of different metal ionsin the field-effect electron mobility in InAs/S²⁻ NC film;

FIG. 41A generally illustrates output characteristics, I_(D) vs. V_(DS)as a function of gate voltage V_(G) for FET devices using K₂Te cappedCdSe NC films (CdTe/Te²⁻/K⁺), where data was obtained using bottomgate/bottom source-drain FET device geometry (Ti/Au source and drainelectrodes, channel length=5 μm and width=7800 μm);

FIG. 41B generally illustrates output characteristics, I_(D) vs. V_(DS)as a function of gate voltage V_(G) for FET devices using the same filmstreated with In³⁺ ions (CdTe/Te²⁻/In³⁺) where data was obtained usingbottom gate/bottom source-drain FET device geometry (Ti/Au source anddrain electrodes, channel length=5 μm and width=7800 μm);

FIG. 41C generally illustrates output characteristics, I_(D) vs. V_(DS)as a function of gate voltage V_(G) for FET devices using the same filmstreated with In³⁺ ions (CdTe/Te²⁻/In³⁺), where data was obtained usingbottom gate/bottom source-drain FET device geometry (Ti/Au source anddrain electrodes, channel length=5 μm and width=7800 μm);

FIG. 41D is a plot of the output characteristics for an n-type FET usingCdTe NCs capped with In₂Se₄ ²⁻ ions (CdTe/In₂Se₄ ²⁻), where data wasobtained using bottom gate/top source-drain FET geometry (Al electrodes,channel length=150 μm and width=1500 μm);

FIG. 41E is a plot of transfer characteristics, I_(D) vs. V_(G) in thesaturation limit for FET devices made of CdTe/Te²⁻/K⁺, where data wasobtained using bottom gate/bottom source-drain FET device geometry(Ti/Au source and drain electrodes, channel length=5 μm and width=7800μm);

FIG. 41F is a plot of transfer characteristics, I_(D) vs. V_(G) in thesaturation limit for FET devices made of CdTe/Te²⁻/In³⁺ NCs, where datawas obtained using bottom gate/bottom source-drain FET device geometry(Ti/Au source and drain electrodes, channel length=5 μm and width=7800μm);

FIG. 41G is a plot of transfer characteristics, I_(D) vs. V_(G) in thesaturation limit for FET devices made of CdTe/Te²⁻/In³⁺ NCs, where datawas obtained using bottom gate/bottom source-drain FET device geometry(Ti/Au source and drain electrodes, channel length=5 μm and width=7800μm);

FIG. 41H is a plot of transfer characteristics, I_(D) vs. V_(G) in thesaturation limit for FET devices made of CdTe/In₂Se₄ ²⁻ NCs, where datawere obtained using bottom gate/top source-drain FET geometry (Alelectrodes, channel length=150 μm and width=1500 μm);

FIG. 42 is a plot of magnetization (M) vs field (H) plots for K₂S cappedCdSe NCs, before (CdSe/S²⁻/K⁺) and after (CdSe/S²⁻/Mn²⁺) treatment withMn²⁺ ions, where the upper-left inset shows M-H data for CdSe/S²⁻/Mn²⁺in the low-field region, and the bottom-right inset shows ZFC and FCcurves measured at 100 Oe for CdSe/S²⁻/Mn²⁺ NCs;

FIG. 43 is a plot of X-band EPR spectrum of CdSe/S²⁻/Mn²⁺ NCs measuredat 4.6K, where the inset shows the variation of EPR signal intensity at305.9 mT during on/off illumination cycles;

FIG. 44A is a plot of a comparison of Mn K-edge XANES spectra for Mn²⁺treated K₂S-capped CdSe NCs (CdSe/S²⁻/Mn²⁺) with standard compoundscontaining Mn in different oxidation states;

FIG. 44B is a plot of magnitude of the Fourier transformed Mn K-edgeEXAFS data and the fit for CdSe/S²⁻/Mn²⁺ NCs;

FIG. 44C is a plot of a comparison of the S-edge XANES spectra forCdSe/S²⁻/K⁺ and CdSe/S²⁻/Mn²⁺ NCs with XANES spectra of K₂S and MnS usedas standards;

FIG. 45A is a schematic of CdSe/S²⁻ NC surface with coordinated Pt²⁺ions showing that electrons supplied by NC can drive proton reduction;

FIG. 45B is a plot of cyclic voltammogramms measured for CdSe/S²⁻/K⁺ andCdSe/S²⁻/Pt²⁺ NC films in anhydrous acetonitrile in presence of 0.01 M[H(DMF)]OTf as a proton source and 0.1 M tetrabutylammonium perchlorateas an electrolyte;

FIG. 45C is a plot of cyclic voltammogramms measured for CdSe/S²⁻/K⁺ andCdSe/S²⁻/Pt²⁺ NC films in aqueous solution buffered at pH 6.5 with aphosphate buffer. The inset shows onsets of hydrogen reduction processfor different electrodes;

FIG. 46 illustrates FTIR spectra of original 4.2 nm CdSe NCs capped withoctadecylphosphonic acid (red line) and after the exchange of theorganic ligands with K₂S (blue line);

FIG. 47 illustrates the variation of ζ-potential of K₂S capped CdSe NCsafter treatment with various metal nitrates, where the concentrations ofmetal ions were adjusted such that the total ionic strength was 15 mMfor all solutions;

FIG. 48 is a plot comparison of PL spectra of Sn₂S₆ ⁴⁻ capped CdSe NCsbefore and after Cd²⁺ treatment, where corresponding ζ-potential valuesare shown in figure legend;

FIG. 49 is a plot of UV-visible absorption spectra of solutions of K₂Scapped CdSe NCs in formamide, before and after treatment with differentmetal ions;

FIG. 50 is a plot of PL spectra of K₂S capped CdSe/ZnS NC dispersion inFA, before and after treatment with different cations, where the PLintensity was normalized to the absorbance of colloidal solutions at theexcitation wavelength (450 nm);

FIG. 51 is a schematics showing agglomeration of CdSe/S²⁻ NCs due tolinking of NCs by Cd²⁺ ions;

FIG. 52 is a plot of powder X-ray diffraction patterns for Cd²⁺ treatedS²⁻ capped CdSe (CdSe/S²⁻/Cd²⁺) NCs at room temperature and afterannealing the sample at 200° C. for 30 minutes;

FIG. 53 is a plot showing transfer characteristics for the field-effecttransistors with FET channel assembled from CdSe/S²⁻/K⁺ NCs (black line)and CdSe/S²⁻/Cd²⁺ NCs (red line). The I_(D) vs. V_(G) curves weremeasured at V_(DS)=3V, where channel length is 150 μm, width is 1500 μm;

FIG. 54A is a plot of output characteristics (I_(D) vs. V_(DS)) as afunction of gate voltage V_(G) for FET devices made of CdSe/S²⁻/K⁺ NCsolids treated with Zn²⁺;

FIG. 54B is a plot of output characteristics (I_(D) vs. V_(DS)) as afunction of gate voltage V_(G) for FET devices made of CdSe/S²⁻/K⁺ NCsolids treated with In³;

FIG. 54C is a plot of output characteristics (I_(D) vs. V_(DS)) as afunction of gate voltage V_(G) for FET devices made of CdSe/S²⁻/K⁺ NCsolids treated with Mn²⁺;

FIG. 54D is a plot of transfer characteristics, (I_(D) vs. V_(G) atV_(DS)=3V) for FET devices made of CdSe/S²⁻/K⁺ NC solids treated withZn²⁺;

FIG. 54E is a plot of transfer characteristics, (I_(D) vs. V_(G) atV_(DS)=3V) for FET devices made of CdSe/S²⁻/K⁺ NC solids treated withIn³;

FIG. 54F is a plot of transfer characteristics, (I_(D) vs. V_(G) atV_(DS)=3V) for FET devices made of CdSe/S²⁻/K⁺ NC solids treated withMn²⁺;

FIG. 55A plots output characteristics (I_(D) vs. V_(DS)) as a functionof gate voltage V_(G) for FET devices made of InAs S²⁻/K⁺ NC solidstreated with Cd²⁺ ions (InAs/S²⁻/Cd²⁺);

FIG. 55B plots output characteristics (I_(D) vs. V_(DS)) as a functionof gate voltage V_(G) for FET devices made of InAs S²⁻/K⁺ NC solidstreated with Mn²⁺ ions (InAs/S²⁻/Mn²⁺);

FIG. 55C plot x transfer characteristics (I_(D) vs. V_(G) at V_(DS)=3V)for InAs/S²⁻/Cd²⁺;

FIG. 55D plots transfer characteristics (I_(D) vs. V_(G) at V_(DS)=3V)for InAs/S²⁻/Cd^(2+;)

FIG. 56A is a plot of transfer characteristics (I_(D) vs. V_(G) atV_(DS)=3V) for Li₂S capped CdSe NCs before (CdSe/S²⁻/Li⁺) and aftertreatment with Cd²⁺ ions (CdSe/Li₂S/Cd²⁺);

FIG. 56B is a plot of transfer characteristics at V_(DS)=3V for K₂Secapped CdSe NC before (CdSe/Se²⁻/K⁺) and after treatment with Cd²⁺ ions(CdSe/K₂Se/Cd²⁺);

FIG. 57A is a plot of output characteristics (I_(D) vs. V_(DS));

FIG. 57B is a plot transfer characteristics (I_(D) vs. V_(G)) for Cd²⁺treated S²⁻ capped CdSe (CdSe/S²⁻/Cd²⁺) NC film annealed at 300° C. for10 seconds using rapid thermal annealing;

FIG. 58 is a plot of transfer characteristics (I_(D) vs. V_(G) atV_(DS)=−30V) for K₂Te capped CdTe NC using FET geometry bottom-gate andtop source/drain gold electrodes;

FIG. 59A is a plot of transfer characteristics (I_(D) vs. V_(G)) atV_(DS)=−30V using bottom-gated and bottom source/drain electrodesgeometry (Ti/Au electrodes, channel length=5 μm, width=7800 μm) for K₂Tecapped CdTe NC treated with Cd²⁺ ions (CdTe/Te²⁻/Cd²⁺);

FIG. 59B is a plot of transfer characteristics (I_(D) vs. V_(G)) atV_(DS)=+30V using bottom-gated and bottom source/drain electrodesgeometry (Ti/Au electrodes, channel length=5 μm, width=7800 μm) for K₂Tecapped CdTe NC treated with Cd²⁺ ions (CdTe/Te²⁻/Cd²⁺);

FIG. 60A is a plot of output characteristics (I_(D) vs. V_(DS));

FIG. 60B is a plot of transfer characteristics (I_(D) vs. V_(G)) In₂Se₄²⁻ capped CdTe NCs (CdTe/In₂Se₄ ²⁻/N₂H₅ ⁺) using bottom gate/topsource-drain geometry with Au source and drain electrodes;

FIG. 61 is an illustration of powder X-ray diffraction patterns for K₂Scapped CdSe NCs before (CdSe/S²⁻/K⁺) and after (CdSe/S²⁻/Mn²⁺) treatmentwith Mn²⁺ ions;

FIG. 62A is a plot of temperature dependence of magnetic susceptibilityfor Mn²⁺ treated S²⁻ capped 4.5 nm InAs NCs (InAs/S²⁻/Mn²⁺), where thelinear dependence is expected in χ_(exp)T vs. T coordinates ifχ_(exp)=C/(T−θ)+χ_(TIP), where χ_(exp) is the magnetic susceptibilitymeasured in experiment, χ_(TIP) is temperature-independent component ofthe magnetic susceptibility, C is the Curie constant, θ is the Curietemperature and T>>θ;

FIG. 62B is a plot of temperature dependence of magnetic susceptibilityfor Mn²⁺ treated S²⁻ capped 4.2 nm CdSe NCs (CdSe/S²⁻/Mn²⁺) plotted inCurie-Weiss coordinates, where the linear dependence is expected inχ_(exp)T vs. T coordinates if χ_(exp)=C/(T−θ)+χ_(TIP), where χ_(exp) isthe magnetic susceptibility measured in experiment, χ_(TIP) istemperature-independent component of the magnetic susceptibility, C isthe Curie constant, θ is the Curie temperature and T>>θ;

FIG. 62C is a modified Curie-Weiss plot for CdSe/S²⁻/Mn²⁺ NCs confirmingthe presence of temperature-independent paramagnetism, where the lineardependence is expected in χ_(exp)T vs. T coordinates ifχ_(exp)=C/(T−θ)+χ_(TIP), where χ_(exp) is the magnetic susceptibilitymeasured in experiment, χ_(TIP) is temperature-independent component ofthe magnetic susceptibility, C is the Curie constant, θ is the Curietemperature and T>>θ;

FIG. 63 is a plot of normalized EPR spectra of a central sextet aftersubtracting broad signals showing that the EPR parameters are the samefor Mn²⁺ treated S²⁻ capped CdSe NCs (InAs/S²⁻/Mn²⁺) and Mn²⁺ treatedS²⁻ capped InAs NCs (InAs/S²⁻/Mn²⁺), where spectra were measured at4.6K;

FIG. 64 is a plot of variation of the EPR signal intensity at 305.9 mTduring light on/off cycle, where gray and black lines were obtainedafter illuminating the sample with a Xe-lamp using a combination ofIR-blocking filter and a long-pass filter with cutoff at 700 nm and 400nm, respectively;

FIG. 65 is a plot comparison of Mn pre-edge (K-edge) XANES spectra forMn²⁺ treated K₂S-capped CdSe NCs (CdSe/S²⁻/Mn²⁺) with MnS and MnO bulkstandard samples;

FIG. 66 is a plot of the real part of the Fourier transformed Mn K-edgeEXAFS data and model fit for of Mn²⁺ treated K₂S capped CdSe(CdSe/S²⁻/Mn²⁺) NCs, the fit parameters are given in Table 2;

FIG. 67 is a comparison of S K-edge XANES for K₂S capped CdSe NCs before(CdSe/S²⁻/K⁺) and after (CdSe/S²⁻/Mn²⁺) Mn²⁺ treatment along withdifferent standards;

FIG. 68 is a plot of the S-edge XANES for K₂S capped CdSe NCs(CdSe/S²⁻/K⁺) after Mn²⁺ treatment (CdSe/S²⁻/Mn²⁺), and a linearcombination fit using XANES data for CdSe/S²⁻/K⁺ NCs and MnS;

FIG. 69 is a plot of UV-visible absorption spectra of ITO-covered glasselectrodes coated with thin films of K₂S capped CdSe NCs before (black)and after (blue) dipping in 0.1 M solutions of K₂[PtCl₄] in FA, wherespectra were taken before (solid) and after (dashed) electrochemicalcycling in acetonitrile between −1.7 V and 0 V vs. Ag⁺/Ag referenceelectrode at 100 mV/s;

FIG. 70 is a plot of cyclic voltammogramms measured for CdSe/S²⁻/K⁺ andCdSe/S²⁻/Pt²⁺ NC films in aqueous solution buffered at pH 6.5 with aphosphate buffer, where the inset shows onsets of hydrogen reductionprocess for different electrodes;

FIG. 71A is a plot of cyclic voltammogramms measured for bare glassycarbon (GC) electrodes before (black curve) and after (blue curve)treatment with 0.1 M K₂PtCl₄ solution in FA for 30 min; and

FIG. 71B is a plot of cyclic voltammogramms measured for bare glassycarbon electrode (black curve) and films of TDPA-capped CdSe NCs before(blue curve) and after (red curve) treatment with 0.1M K₂PtCl₄ solutionin FA for 30 min, where all measurements were carried out in aqueoussolutions buffered at pH 6.5 with a phosphate buffer. and the potentialscan rate was 100 mV/s.

FIG. 72 shows in (a) typical metal complexes and in (b) a nanocrystalwith surface ligands.

While the disclosed compositions and methods are susceptible ofembodiments in various forms, there are illustrated in the drawings (andwill hereafter be described) specific embodiments of the invention, withthe understanding that the disclosure is intended to be illustrative,and is not intended to limit the invention to the specific embodimentsdescribed and illustrated herein.

DETAILED DESCRIPTION OF THE INVENTION

The compositions and methods described herein generally relate to a newcolloidal material, a composite, made from nanometer scale particles andsoluble precursors to inorganic matrices. The colloidal material can befacilely produced from single or multi-component mixtures of colloidalparticles. These new colloidal materials and the general methodsdescribed herein may reduce the time, expense, and uncertainty in themanufacture of advanced materials. Disclosure of U.S. patent applicationSer. No. 13/266,079 (published as U.S. Patent Application No.2012/104325 and as International Patent Publication No. WO 10/124212) isincorporated by reference in its entirety.

Colloidal particles, from which colloidal materials can be produced, arediscrete particles and are generally suspendable in at least onesolvent. Often colloidal particles have sizes ranging from the nanometerscale to the micron scale and commonly exist as mixtures of colloidalparticles with broad size ranges. One subset of colloidal particles arenanoparticles, specifically those colloidal particles where at least thecross-sections of the particle in two dimensions are between about 1 andabout 1000 nanometers (nm). Nanoparticles can be produced in a largevariety of morphologies and sizes all of which are applicable herein.Nonlimiting examples of the morphologies of nanoparticles includenanocrystals, nanorods, nanoplates, nanowires, and dendriticnanomaterials. Within each morphology there is an additional largevariety of shapes available, for example nanocrystals can be produced inspheres, cubes, tetrahedra, octahedra, icosahedra, prisms, cylinders,wires, branched and hyperbranched morphologies and the like. Themorphology and the size of the nanoparticles do not inhibit the generalmethod for making colloidal materials described herein; specifically theselection of morphology and size of the nanoparticle allows for thetuning and control of the properties of the colloidal material, as willbecome clear.

Non-limiting examples of nanoparticles applicable herein include: AlN,AlP, AlAs, Ag, Au, Bi, Bi₂S₃, Bi₂Se₃, Bi₂Te₃, CdS, CdSe, CdTe, Co, Cu,Fe, FeO, Fe₂O₃, Fe₃O₄, FePt, GaN, GaP, GaAs, GaSb, GaSe, Ge, HgS, HgSe,HgTe, InN, InP, InSb, InAs, Ni, PbS, PbSe, PbTe, Pd, Pt, Ru, Rh, Si, Sn,ZnS, ZnSe, ZnTe, and mixtures thereof. Examples of applicablenanoparticles include core/shell nanoparticles like CdSe/CdS, CdSe/ZnS,InP/CdSe, InP/ZnS, InP/ZnSe, InAs/CdSe, and InAs/ZnSe; nanorods likeCdSe, core/shell nanorods like CdSe/CdS; nano-tetrapods like CdTe, andcore/shell nano-tetrapods like CdSe/CdS.

Often nanoparticles are synthesized in such a manner that the resultingnanoparticle is capped, or coated, in a stabilizing organic material, anorganic capping agent. One typical example of an organic capping agentis trioctylphosphine oxide (TOPO), used in the manufacture of CdSe. TheTOPO capping agent prevents the agglomeration of the nanoparticle duringand after the synthesis of the particle, additionally the long organicchains radiating from the capping agent on the surface of thenanoparticle assist in the suspension and/or solubility of thenanoparticle in a solvent. A second type of organic capping agent is aorganic thiol, often used in the manufacture of silver or goldnanoparticles. These organic thiols range from simple long chain alkanethiols, like dodecane thiol, to complex monothiols. The long organicchain of the organic thiol capping agent, as with the TOPO cappingagent, assists in the suspension and/or solubility of the cappednanoparticle. Other known capping agents include long-chain aliphaticamines, long-chain aliphatic phosphines, long-chain aliphatic carboxylicacids, long-chain aliphatic phosphonic acids and mixtures thereof.

The present disclosure provides techniques for replacement of theorganic capping agents with inorganic capping agents. As used herein,inorganic capping agents refer to those soluble reagents free of organicfunctionality that displace organic capping agents from nanoparticlesand wherein the inorganic capped nanoparticle is dispersible, that issuspendable and/or soluble. One aspect of the technique for replacingthe organic capping agents with inorganic capping agents is the completedisplacement of the organic capping agents from the nanoparticle andreplacement with the inorganic capping agent.

Inorganic capping agents can be neutral or ionic, and can be discretespecies, linear or branched chains, or two-dimensional sheets. Ionicinorganic capping agents are commonly referred to as salts, a pairing ofa cation and an anion, and the portion of the salt specifically referredto as an inorganic capping agent is the ion that displaces the organiccapping agent and caps the nanoparticle. Often an inorganic ion ispaired with an ion that has organic functionality; the paired ion thatdoes not displace organic capping agents is referred to herein as acounter ion. The counter ion can affect the solubility and reactivity ofthe inorganic capping agent as well as the inorganic capped nanomaterialbut the great variability of counter ions allows for their facilereplacement and a balance of desired properties.

Inorganic capped nanoparticles differ from core/shell nanoparticles.Core/shell nanoparticles are those particles where, often, onenanocrystal is subjected to a treatment with a reagent such that acrystalline layer forms over all or a portion of the originalnanocrystal. Core/shell nanoparticles are commonly designated throughthe simple formula of (core composition)/(shell composition), forexample CdSe/CdS has a CdSe core and a CdS shell. The crystalline layer,the shell, generally forms over the entire nanocrystal but as usedherein core/shell nanoparticles refers to those nanoparticles where atleast one surface of the nanocrystal is coated with a crystalline layer.While the inorganic capping agents may form ordered arrays on thesurface of a nanocrystal these arrays differ from a core/shellcrystalline layer because the thickness of the array is dependent on thesize of the inorganic capping agent not the concentration of reagentprovided or the growth time of the layer, because, without being boundby theory, the ordering of the inorganic capping agent array on thenanocrystal is believed to be dependent on the exposed crystal face ofthe nanocrystal not the crystal structure of the crystalline (shell)layer, and because the interaction with the face of the nanocrystal isindependent of the crystal structure of the crystalline (shell) layerthere is no crystal structure mismatch between the inorganic cappingagent and the crystal faces of the nanocrystal.

The disclosed colloidal particles, e.g. inorganic capped nanoparticles,are soluble and/or suspendable in a solvent. Typically, the purificationof chemicals requires some isolation procedure and for inorganicmaterials this procedure is often the precipitation of the inorganicproduct. The precipitation of inorganic products permits one of ordinaryskill to wash the inorganic product of impurities and/or unreactedmaterials. The isolation of the precipitated inorganic products thenallows for the selective application of those materials.

Moreover, the disclosed colloidal particles are isolable anddispersible, soluble or suspendable, in a variety of solvents.Manufacturing techniques, environmental and/or work-place rules oftenrequire the application of certain solvents. Colloidal materialsdispersible in a variety of solvents are applicable for a broader usethan those colloidal materials that require specific, toxic,environmentally hazardous, or costly solvents.

Solvents applicable to the isolation of the disclosed colloidalparticles include but are not limited to: 1,3-butanediol, acetonitrile,ammonia, benzonitrile, butanol, dimethylacetamide, dimethylamine,dimethylethylenediamine, dimethylformamide, dimethylsulfoxide, dioxane,ethanol, ethanolamine, ethylenediamine, ethyleneglycol, formamide,glycerol, methanol, methoxyethanol, methylamine, methylformamide,methylpyrrolidinone, pyridine, tetramethylethylenediamine,triethylamine, trimethylamine, trimethylethylenediamine, and water.

The above-described colloidal particles are made by mixing thenanoparticle with the inorganic capping agent in accordance with thetechniques described herein. Often the mixing of the nanoparticle withthe inorganic capping agent causes a reaction of the nanoparticle withthe inorganic capping agent and rapidly produces insoluble andintractable materials. Herein a mixture of immiscible solvents is usedto control the reaction of the nanoparticle with the inorganic cappingagent. Immiscible solvents facilitate a rapid and complete exchange ofthe organic capping agents with the inorganic capping agents and preventthe production of the common intractable/insoluble materials.

Generally, the inorganic capping agent is dissolved in a polar solventwhile the nanoparticle is dissolved in an immiscible, generallynon-polar, solvent. These two solutions are then combined in a singlevessel. Non-limiting examples of polar solvents include 1,3-butanediol,acetonitrile, ammonia, benzonitrile, butanol, dimethylacetamide,dimethylamine, dimethylethylenediamine, dimethylformamide,dimethylsulfoxide, dioxane, ethanol, ethanolamine, ethylenediamine,ethyleneglycol, formamide, glycerol, methanol, methoxyethanol,methylamine, methylformamide, methylpyrrolidinone, pyridine,tetramethylethylenediamine, triethylamine, trimethylamine,trimethylethylenediamine, water, and mixtures thereof. Non-limitingexamples of non-polar solvents include pentane, pentanes, cyclopentane,hexane, hexanes, cyclohexane, heptane, octane, nonane, decane, dodecane,hexadecane, benzene, 2,2,4-trimethylpentane, toluene, petroleum ether,ethyl acetate, diisopropyl ether, diethyl ether, carbon tetrachloride,carbon disulfide, and mixtures thereof; provided that the non-polarsolvent is immiscible with the polar solvent. Other immiscible solventsystems that are applicable include aqueous-fluorous, organic-fluorous,and those using ionic liquids.

Without wishing to be bound by theory, it is thought that thenanoparticle reacts with the inorganic capping agent at or near thesolvent boundary, the region where the two solvents meet, and a portionof the organic capping agent is exchanged/replaced with the inorganiccapping agent, that is the inorganic capping agent displaces an organiccapping agent from a surface of the nanoparticle and the inorganiccapping agent binds to the surface of the nanoparticle. Without wishingto be bound by theory, it is thought that this process continues untilan equilibrium is established between the inorganic capping agent on ananoparticle and the free inorganic capping agent. Preferably, theequilibrium favors the inorganic capping agent on a nanoparticle,although other equilibria are applicable for making mixed colloidalparticles. The herein described immiscible solvent method of formingnanoparticles capped with inorganic capping agents has the added benefitof providing for the extraction of the organic capping agents from thepolar-solvent-soluble reaction product.

The sequential addition of inorganic capping agents on a nanoparticle isavailable under the disclosed method. Depending, for example, uponconcentration, nucleophilicity, capping agent to nanoparticle bondstrength, and crystal face dependent capping agent to nanoparticle bondstrength, the inorganic capping of the nanoparticle can be manipulatedto yield hereto unknown combinations. For example, assume that ananoparticle has two faces, face A and face B, and that the bondstrength of the organic capping agent to face A is twice that of thebond strength to face B. The organic capping agents on face B will bepreferentially exchanged when employing the herein describedmethodology. Isolation and reaction of this intermediate species, havingorganic and inorganic capping agents, with a second inorganic cappingagent will produce a colloidal particle with a first inorganic cappingagent on face B and a second inorganic capping agent on face A.Alternatively, the preferential binding of inorganic capping agents tospecific nanoparticle faces can yield the same result from a singlemixture of multiple inorganic capping agents.

Another aspect of the disclosed method is the possibility of a chemicalreactivity between inorganic capping agents. This inorganic cappingagent reactivity can be employed to make colloidal particles wherein,for example, the nanoparticle is capped by an inorganic capping agent.For example, a first inorganic capping agent bound to the surface of ananocrystal and reactive with a second inorganic capping agent candirectionally react with the second inorganic capping agent. Thismethodology provides for the synthesis of colloidal particles that couldnot be selectively made from a solution of nanoparticle and inorganiccapping agents. The interaction of the first inorganic capping agentwith the nanoparticle controls both the direction and scope of thereactivity of the first inorganic capping agent with the secondinorganic capping agent. Furthermore, this methodology controls whatpart of the new inorganic capping agent binds to the nanocrystal.Presumably, the result of the addition of a combined-inorganic cappingagent capping to a nanocrystal by other methods will produce a randomarrangement of the combined-inorganic capping agent on the nanocrystal.

Yet another method of making colloidal particles involves the mixing ofa nanoparticle in a non-polar organic solvent with a purified colloidalparticle in a polar organic solvent. In this example, the colloidalparticle in the polar solvent is the inorganic capping agent. Thesemethodologies can form colloidal particle capped nanoparticles and othervariations on the capping architecture.

Still another aspect of the current disclosure is the bridging ofcolloidal particles with metal ions. Herein bridging means that one ormore metal ions individually bind, ionically or covalently, bindingsites on the exterior of a plurality of colloidal particles. Preferably,binding sites are parts of the inorganic capping agent that is disposedperpendicular to a surface on the nanoparticle. Understanding that thebridging between two colloidal particles can involve a plurality ofmetal ions, for descriptive purposes the binding of a single metal ionbetween two colloidal particles is exemplified herein. As describedabove, inorganic capping agents include anionic and neutral cappingagents, when for example, anionic inorganic capping agents are bound tothe surface of a nanoparticle the anionic charge associated with theinorganic capping agent remains (providing a theoretical basis forelectrostatic repulsion between colloidal particles having anionicinorganic capping agents). The addition of a cationic ion, preferably acationic metal ion, still more preferably a polycationic (wherein thecharge on the metal ion is greater than 1) metal ion to the colloidalparticle can produce a colloidal particle with the cationic ion bound toa surface of the colloidal particle (herein, an inorganic cappingagent). Preferably, the cationic ion additionally binds to the surfaceof a second colloidal particle thereby bridging between the twocolloidal particles. Preferably, the cationic ion is a transition metalion, a main group ion, a lanthanide ion, or an actinide ion. Morepreferably, the cationic ion is selected from those ions that can impartadvanced electronic, magnetic (e.g., Mn²⁺, Co²⁺), or photophysicalproperties on the bridged colloidal particles. Still more preferably thecation ion is Pt²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Mn³⁺, Eu²⁺, Fe²⁺, Fe³⁺, Co²⁺,Ge⁴⁺, Cu²⁺, Cu⁺, Pb²⁺, Bi³⁺, Sb³⁺, In³⁺, Ga³⁺, Au⁺, Au³⁺, Ag⁺, Sn²⁺,Sn⁴⁺, or Hg²⁺. When the addition of the cationic ion to the colloidalparticle is conducted on a scale wherein there is a multiplicity ofcolloidal particles, the binding and bridging of the colloidal particlesis termed a polycondensation reaction. Herein, a controlledpolycondensation reaction yields colloidal materials. Moreover, theselection of cationic ion and polycondensation reaction conditionsallows for control of the three-dimensional structure of the colloidalmaterial. Without being bound by theory, these polycondensationreactions are envisioned as behaving analogously to molecularself-assembled three-dimensional arrays.

Another aspect of the current disclosure is the isolation of thecolloidal particles as colloidal materials. As used herein, colloidalmaterials are samples of the herein described colloidal particle in asolid form. The form can be ordered or disordered. The nanoparticle mayhave a crystalline, that is, ordered internal structure but theprecipitation of the colloidal particles may produce a random(disordered) arrangement of particles. Alternatively, the controlleddissolution or spontaneous self-assembly of the colloidal particles canyield ordered arrays of particles.

The careful deposition of thin layers of colloidal arrays yields orderedarrays dependent on the possible packing of the colloidal particle inthree dimensional space. As non-limiting examples, the deposition ofcolloidal particles of nanocrystals that are roughly spherical hasyielded hexagonal close-packed arrays of colloidal particles or cubicclose-packed arrays of colloidal particles. Such selective deposition ofcolloidal particles has advanced materials applications for which theselection of the nanoparticle and the ordering of the three dimensionalarray can change the physical and electronic characteristics of thematerial.

The deposition of layers of the colloidal particles to make colloidalmaterial thin films is another aspect of the current disclosure.Depending on the desired application or the available resources,multiple film deposition methodologies are available. One method that isapplicable to form colloidal material thin films is a reverseLangmuir-Blodgett technique. Herein the insolubility of the colloidalparticle in nonpolar solvents permits the monolayer deposition of filmsof the colloidal particle on a substrate. Multiple application of theLangmuir-Blodgett technique can be employed to grow multilayer films ofthe colloidal particle or mixed layer films of colloidal particles.

A second applicable technique for the deposition of layers of thecolloidal particles to make colloidal material thin films is spincoating. Here a solution of the colloidal particle is applied to asubstrate and the substrate and solution are rapidly rotated to leave athin layer of the solution on the substrate, this solution is then driedleaving the colloidal material thin film. Herein, the use of multiplesolvents described above provide a hereto before unknown control in themanufacture of these films. The wetting of the substrate by thecolloidal particle solution is an important factor in achieving uniformthin films and the ability to apply the colloidal particle in a varietyof different solvents enhances the commercial applicability of thistechnique. One method to achieve uniform wetting of the substratesurface is to match the surface free energy of the substrate with thesurface tension of the liquid (colloidal particle solution). The perfectwetting of a substrate by a colloidal particle solution will yield auniform colloidal material thin film on the substrate.

Additional techniques for the deposition of layers of the colloidalparticles to make colloidal material thin films include spray coating,spray pyrolysis, dipping, sputtering, printing, and the like andcombinations of spray coating, spray pyrolysis dipping, sputtering,printing and the like with spin coating.

Colloidal material can additionally be produced through a combination ofone of the above processes with the bridging of the colloidal particleswith cationic ions. One example of this combination method is thealternating dipping of a substrate into a solution of colloidalparticles and a solution of cationic ions. Preferably, the substrate hasa sufficient affinity for the first material, whether colloidalparticles or cationic ions, that a layer of the first material uniformlydeposits on the surface of the substrate. Examples of substrates whereinthe colloidal particle would be the first material include but are notlimited to mono or multilayers of copper, silver, gold, nickel,palladium, platinum, alloys, and amalgams thereof. Moreover, when acolloidal particle has sulfur, selenium, and or tellurium atoms on theexposed surface (the outermost surface of the colloidal particle)thiophilic substrates are preferentially employed. Herein, thiophilicmeans having a thermodynamic preference for binding to sulfur, selenium,and/or tellurium. Examples of thiophilic substrates include monolayers,multilayers, or bulk amounts of silver, gold, and other known thiophilicsubstrates. Another preferred substrate applicable when the firstmaterial is a colloidal particle is a positively charged substrate.Substrates made from glass, sapphire, quartz, silicon, or other solidmaterials can be manipulated to have a positive charge through manyknown methods. For example, the chemical treatment of a substrate withan amine functionalized siloxane can produce a positively chargedsurface on the substrate. Alternatively, the substrate can be coatedwith a cationic polymer through any of the known methods for formingpolymer coatings on substrates. Examples of polymers applicable to coatsubstrates to form the positive charged substrate include the knownpolyquaternium polymers. One of the preferred cationic polymers ispoly(diallyldimethylammonium) chloride (polyquaternium-6 (CAS #:26062-79-3)). A typical procedure for this alternating dip coatingprocess is to repetitively, dip the substrate in a first solution, rinsethe substrate, dip the substrate in a second solution, and rinse thesubstrate. Typically, as described above, the substrate prior to anydipping preferentially binds the colloidal particle thereby the firstsolution is a solution having the colloidal particle dispersed therein.Additional combination procedures are available, for example first spincoating a substrate with a solution of a colloidal particle then dippingthe substrate in a solution of a cationic ion and then repeating.

Another technique wherein the ability to select the solvent in which thecolloidal material is dissolved and from which it is deposited isink-jet deposition. Similar to the necessary balance in the wetting of asubstrate, ink-jet deposition often requires the ability to adjustliquid surface tensions to applicable ranges. One easy method foradjusting the surface tension of a liquid containing colloidal particlesis the blending or mixing of a multiplicity of solvents.

Yet another application of the present disclosure is the deposition ofcolloidal particles to make non-thin film solids. Herein, the describedcolloidal particles are deposited by bulk techniques to yield threedimensional solids. Known bulk deposition techniques include pressing ofpowders, growth of three-dimensional ordered arrays, painting, printing,and the like.

Still another aspect of the techniques described above for theproduction or deposition of colloidal materials is the production ordeposition of mixed, colloidal particle, solids. These hetero-colloidalmaterials comprise a plurality of colloidal particles in the resultingsolid material. Similar to the disclosure above, multiple techniques canbe used to produce hetero-colloidal materials. Non-limiting examplesinclude, mixing of colloidal particles in a solvent followed bydeposition of a hetero-colloidal material, alternating deposition ofcolloidal material films to produce a layered hetero-colloidal material,multispray coating of a substrate, and blending of colloidal materialsolids followed by pressing into a cohesive material.

One example of the hetero-colloidal materials described herein is abinary superlattice. Binary superlattices are those organized structureswherein the three-dimensional packing of two different nanoparticlesproduces an ordered structure. Multiple physical and chemicalcharacteristics can facilitate the production of binary superlattices,for example, nanoparticle size, nanoparticle shape, and nanoparticleCoulombic potential. This assembly of two different colloidal particlesinto a binary superlattice is a general and inexpensive method toproduce multiple hetero-colloidal materials with precise control of thechemical composition and physical properties. D. V. Talapin, et al.,“Dipole-dipole interactions in nanoparticle superlattices.” Nano Letters2007, 7, 1213-1219.

Yet another aspect of the current disclosure is the thermal treatment ofthe herein described colloidal materials. As discussed above, many ofthe inorganic capping agents are precursors to inorganic materials(matrices) and low-temperature thermal treatment of these inorganiccapping agents provides a gentle method to produce crystalline filmsfrom these materials. The thermal treatment of colloidal materialsyields, for example, ordered arrays of nanoparticles within an inorganicmatrix, hetero-alloys, or alloys. A multiplicity of thermally treatedcolloidal materials are available from the disclosed colloidalparticles.

As used herein, colloidal matrices are ordered arrays of nanoparticleswithin an inorganic matrix. Generally, the inorganic matrix has largercrystal domains than the nanoparticles and, importantly, separates thenanoparticles. The inorganic matrix can be a glass, a solid, or acrystalline material. Additionally, the order of the array ofnanoparticles can be short range or long range. Very dilute samples ofnanoparticles in the inorganic matrix are expected to have lowerrelative ordering than concentrated samples wherein the nanoparticlesmay be ordered prior to and preferably after the formation of theinorganic matrix.

Additional embodiments of the hetero-colloidal matrix include selectivedeposition of colloidal materials in confined spatial arrangementsfollowed by thermal treatment to form the inorganic matrix. The layered,structured, or patterned deposition of a plurality of colloidalmaterials followed by thermal treatment to form the inorganic matrixcreates solid-state architecture that is not available by other knownmethodologies.

The colloidal matrices can be produced in thin films, films, coatings,solids and/or mixed solids. Moreover, the colloidal matrices can beproduced in bulk, layered, structured, or patterned arrangements on asubstrate. Additionally, the procedure described herein yields colloidalmatrices that effectively preserve the electronic characteristics of thenanoparticle after thermal treatment.

Another embodiment of the materials and methods disclosed herein is analloy made from a nanoparticle and an inorganic capping agent. Alloysare continuous homogeneous phases of a composition and herein alloys areproduced by the thermal treatment of the disclosed colloidal particles.Similar to the colloidal matrices, the formulation of the alloy isdependent on the nanoparticle and inorganic capping agent. Unlike thecolloidal matrices, the formation of the alloy involves the destructivereaction of the inorganic capping agent, and/or optionally additionalreagent(s), with the nanoparticle, herein a destructive reaction meansthe nanoparticle loses at least one aspect of its defining physicalcharacteristic, examples include size, shape, photoactivity,formulation, and the like.

Generally, the formation of an alloy requires some atom mobility duringthe thermal treatment phase and processing conditions can and often doaffect the formation of an alloy. The incomplete alloying of thedisclosed materials, whether purposefully or serendipitously, yields ahetero-alloy. As used herein, hetero-alloys are solid state materialsformed from the thermal treatment of a colloidal material that ischaracterized by a multiplicity of domains, wherein the domains may havedifferent formulations and/or crystal structures and/or crystallinity.Whether a thermal treatment of a colloidal material forms an alloy or ahetero-alloy is often difficult to determine, but, without being boundby theory, it is believed that a lower temperature thermal treatmentlimits atom mobility and therefore prohibits the formation of ahomogeneous phase (alloy).

Yet another embodiment of the materials and methods disclosed herein isthe deposition of colloidal matrices, alloys, or hetero-alloys on asurface to form an advanced material, a printed circuit, a solar cell, athermoelectric cell, a light emitting diode, and the like. The colloidalmaterials disclosed herein are applicable for the printing or depositionof colloidal matrices, alloys, or hetero-alloys through the applicationand heating of the colloidal material on a substrate. Representativeexamples of the application of the disclosed colloidal materials includesputter deposition, electrostatic deposition, spray deposition, spincoating, inkjet deposition, laser printing (matrices), and the like. Analternative method of deposition is the electrochemical deposition of acolloidal matrix from a solution of a colloidal material.

The low temperature formation of the colloidal matrix, alloy, orhetero-alloy makes the disclosed methodology compatible withphotolithographic patterning, for example, wherein a photolithographicapplied substrate mask is removed after the thermal treatment of thecolloidal material.

Another aspect of the disclosed materials and methods is the formationof materials that exhibit enhanced thermoelectric properties; that isthe direct conversion of a thermal gradient to electrical energy.Thermoelectric energy conversion is an all-solid-state effect thatconverts thermal gradients directly to electrical energy without anelectromechanical generator. The derived voltage and power, work, drainsthe heat from the location of the thermal gradient. Materials thatdisplay thermoelectric energy conversion are useful in heat pumps, powergenerators, and thermoelectric coolers. Thermoelectric devices have nomoving parts and therefore have advantages in reliability, silentoperation, high power density, and high efficiency for moderate powerapplications. In addition, thermoelectric materials can be used forcooling by driving current through the material.

The NC can be capped with an inorganic ligand. Contemplated inorganicligands include oxo- and polyoxometalates (POMs). POMs are heavilycharged anions that can accept one or more electrons without significantstructural changes. The delocalization of these electrons within the POMframework makes them candidates in catalysis or related applications.Unlike some other inorganic ligands, most of the oxo-ligands do notreplace the surface organic ligands directly. A two-step ligand exchangeprocedure was used, including the removal of original organic ligandsfollowed by the adsorption of negatively charged inorganic ligands.

In some exemplary embodiments, metal oxide Fe₂O₃ NCs and semiconductorCdSe NCs were used as standard systems. POMs of early transition metalsmolybdenum or tungsten are specifically contemplated, as are Keggin typeand Dawson type POMs. Most Keggin type POMs [X^(n+)M₁₂O₄₀]^((8-n)−) arecommercially available (X═P, Si; M=Mo, W). Dawson type POMs[(X^(n+))₂M₁₈O₆₂]^((16-2n)−), polymolybdate giant wheels {Mo₁₁}₁₄ andknown water oxidation catalystRb₈K₂[{Ru₄O₄(OH)₂(H₂O)₄}(γ-SiW₁₀O₃₆)₂]25H₂O were prepared followingliterature protocols. (Graham et al., Inorg. Chem., 2008, 47: 3679-3686;Geletii et al., Angew. Chem., 2008, 120: 3960-3963; and Rosen et al.,Angew. Chem. Int. Ed., 51:684-689) Simple oxo-molecules were alsoexplored such as VO₄ ³⁻, MoO₄ ²⁻, WO₄ ²⁻, PO₄ ³⁻, AsO₄ ³⁻, HPO₃ ²⁻,H₂PO₂, and the like.

For a typical ligand exchange, a solution of organic-capped NCs in anonpolar solvent (toluene or hexane) were combined with a solution of aninorganic ligand (K₂S, KHS, (NH₄)₂TeS₃, KOH, KNH₂, etc) in formamide(FA). The two-phase mixture containing immiscible layers of FA andnonpolar solvent was stirred for about 10 min, after which completetransfer of NCs from non-polar solvent to FA (FIG. 1a ) was observed.Such exchange can be carried out in different solvents, ruling out aspecial role of FA in colloidal stabilization. For example, S²⁻-cappedCdSe NCs can be easily prepared using (NH₄)₂S aqueous medium or Na₂S andK₂S in dimethylsulphoxide (DMSO). Experimental results suggested thatcolloidal stability of NC dispersions is mainly determined by thesolvent dielectric constant; highly polar FA (ε=106) was found to be thebest solvent, while water (ε=80) and DMSO (ε=47) provided moderatestability. DMF, the least polar in this series (ε=36), did not stabilizethe colloidal dispersions.

In some cases, the ligand exchange can be performed in the followingmanner. For organic ligands removal, either phase transfer or directligand removal can be performed. For a typical phase transfer process,certain amount of nitrosonium tetrafluoroborate (NOBF₄), fluoroboricacid (HBF₄) or trimethyloxonium tetrafluroborate (Me₃OBF₄) was firstdissolved in fresh dimethylformamide (DMF). NCs in fresh hexane wereadded on top of the DMF phase. The resulting mixture was stirredvigorously until NCs were completely transferred into DMF phase. For adirect ligand removal process, NCs were first diluted with excessivesolvent such as hexane, followed by addition of nitrosoniumtetrafluoroborate (NOBF₄), fluoroboric acid (HBF₄) or trimethyloxoniumtetrafluroborate (Me₃OBF₄). The mixture was stirred vigorously until NCswere precipitated. NCs were collected and dispersed into fresh DMF.

To functionalize NCs with oxo-molecules or POMs, they were precipitatedfrom DMF with addition of toluene or toluene/hexane mixture. Certainamount of oxo-molecules or POMs in formamide (FA) was added to NCs inFA. The stabilization of oxo-molecules or POMs capped NCs in FA wasstrongly depending on the pH. NCs treated with NOBF₄ or HBF₄ to removeorganic ligands usually formed very acidic solution in DMF. They neededto be further purified with FA/acetonitrile to increase the pH to avalue suitable for oxo-ligands adsorption. Following this approach, bothCdSe and Fe₂O₃ NCs can be colloidally stabilized with VO₄ ³⁻, MoO₄ ²⁻,WO₄ ²⁻, PO₄ ³⁻, AsO₄ ³⁻, HPO₃ ²⁻, H₂PO₂ etc. Fe₂O₃ NCs can be furtherstabilized with POMs such as Na₃[PMo₁₂O₄₀], K₆[P₂MO₁₈O₆₂] etc (Table 1).Other metal oxide NCs such as ZnO, CoFeO and TiO₂ with some of the POMsare also contemplated (Table 1).

Different characterization techniques were used to confirm thatoxo-molecules or POMs were attached to the NCs surface. Dynamic lightscattering measurements showed that the size of NCs decreased from 15 nmto 13 nm after organic ligands removal and increased around 1-2 nm aftercapping with POMs (FIG. 22A). Zeta potential measurement showed thatboth CdSe and Fe₂O₃ NCs surface charge changed from positive to negativeafter oxo-ligands exchange. In aqueous or similar media, a zetapotential of ±30 mV is regarded as the borderline between stable andunstable colloids. FIG. 23B indicated that CdSe and Fe₂O₃ NCs cappedwith PO₄ ³⁻ were stable in FA. FTIR spectra acquired for both CdSe andFe₂O₃ NCs showed that the majority of the surface organic ligands wereremoved after NOBF₄ or HBF₄ treatment (FIG. 22C, 22D). C—H vibrationcoming from organics from 2500-3300 cm⁻¹ was strongly suppressed. TEMimages of CdSe and Fe₂O₃ NCs capped with various oxo-ligands were takenfrom either FA or N-methylformamide (NFA) (FIGS. 23, 24). In NFA, thespacing between NCs was larger than that in FA possibly due to NFAmolecules also acting as ligands to stabilize NCs.

TABLE 1 NCs Ligands Fe₂O₃ ZnO CoFeO TiO₂ H₃[PMo₁₂O₄₀] Stable Stable H₃[PW₁₂O₄₀] Stable Stable Na₃PMo₁₂O₄₀ Stable Stable Na₆H₂W₁₂O₄₀ StableStable H₄[SiW₁₂O₄₀] Stable (NH₄)₆[H₂W₁₂O₄₀] Unstable K₆[P₂W₁₈O₆₂] StableStable Stable Stable K₆[P₂Mo₁₈O₆₂] Stable Stable Stable Mo₁₅₄ StableStable Stable Rb₈K₂[{Ru₄O₄(OH)₂ Stable (H₂O)₄}(γ-SiW₁₀O₃₆)₂]

To further explore the ligand chemistry, simplified zeta potentialtitration experiments were performed to study the ligand bindingprocess. In a typical experiment, CdSe or Fe₂O₃ NCs were treated withNOBF₄, HBF₄ or Me₃OBF₄ to remove the surface ligands resulting in stablecolloidal solution in DMF (˜5 mg/ml). These NCs were positively chargeddue to unbound metal sites on the surface such as Cd²⁺ or Fe³⁺. For zetapotential titration, 50 μl of positively charged NCs in DMF (5 mg/ml)was added to 1 ml of DMF or FA. Oxo-ligands stock solutions wereprepared at different concentrations such as 3 mM, 5 mM, 10 mM and 15mM. Zeta potential of the colloidal solution was measured with theaddition of oxo-ligands of increasing volume. Ideally, the surfacepositive charge would be slowly neutralized by negatively chargedoxo-ligands until the surfaces of NCs were fully passivated at whichpoint the surface charge would be saturated (FIG. 22E).

To compare the binding capability of different ligands such asoxo-ligands and chalcogenide ligands, K₂S₂O₃, Na₂MoO₄, (NH₄)₂MoS₄ wereselected as typical ligands for CdSe NCs. The electrophoretic mobilitieswere measured using Zetasizer Nano-ZS. Excessive NaBF₄ was added to thesamples to contribute to the total ionic strength. The electrophoreticmobilities were plotted according to the ligands concentration, whichindicated S₂O₃ ²⁻, MoS₄ ²⁻ have stronger binding affinity to CdSe NCscompared to MoO₄ ²⁻ (FIG. 25A). The same experiments were performed onFe₂O₃ NCs (15 nm) as well. Na₃AsS₃, H₃[PMo₁₂O₄₀], Na₂MoO₄, (NH₄)₂MoS₄were chosen as ligands. Similar results were obtained as CdSe NCs (FIG.25B). It showed [PMo₁₂O₄₀]³⁻, MoO₄ ²⁻ binded to Fe₂O₃ NCs stronger thanMoS₄ ²⁻, and AsS₃ ³⁻ which, without being bound by theory, can beexplained by the hard and soft acids and bases (HSAB) principles.

From the experiments of zeta potential titration, pH was an influentialparameter for Fe₂O₃ NCs just after treatment with either NOBF₄ or HBF₄showed strong acidity (˜1.4-1.8 in DMF) while they showed a higher pHaround 3 after treatment with Me₃OBF₄. After washing the originalsolution with different solvents/non-solvents combinations, the pHincreased to around 7-11, and showing a difference in zeta potentialtitration. The solution with a higher pH showed the ligands bindingtrend as expected while the same ligand did not show a strong bindingcapability to NCs in a less acidic solution (FIG. 26).

In the case of CdSe NCs, there was not a huge influence of pH on ligandsbinding capability. Possible reasons could be: pH affects the surfacecharge density of Fe₂O₃ NCs and pH affects the actual binding speciesexisting in the solution.

Chalcogenide and Hydrochalcogenide Ligands (S²⁻, HS⁻, Se²⁻, HSe⁻, Te²⁻and HTe⁻).

FIG. 1B compares the absorption and photoluminescence (PL) spectra of5.5 nm CdSe NCs capped with organic ligands in hexane and S²⁻ in FA. Theexcitonic features in the absorption spectrum of CdSe NCs remainedunchanged after the ligands exchange, implying no changes in size, shapeand size-distribution of CdSe NCs. S²⁻-capped CdSe NCs in FA also showedband-edge photoluminescence with ˜2% QE, which was rather unexpectedsince alkylthiols are well-known PL quenchers for CdSe NCs. In contrastto thiols, charged S²⁻ ligands do not introduce midgap states serving asfast non-radiative recombination channels. CdSe NCs capped with HSligands also retained their band-edge PL (FIG. 7). S²⁻ can replacedifferent kinds of organic ligands such as carboxylates, amines,phosphonic acids, and alkylphosphonic oxide from surface of CdSe NCs.

Regarding FIGS. 1A-1F; (a) Red colored colloidal dispersion of CdSe NCsundergoes the phase transfer from toluene to FA upon exchange of theoriginal organic surface ligands with S²⁻. (b) Absorption and PL spectraof 5.5 nm CdSe NCs capped with organic ligands and S²⁻ ligands dispersedin toluene and FA respectively. (c) FTIR spectra of 5.5 nm CdSe NCs withdifferent combinations of ligands and solvents. (d) NCsize-distributions measured by Dynamic Light Scattering for ˜12 nm CdSeNCs capped with organic ligands and S²⁻ ions. Inset shows TEM image ofthe NCs capped with S²⁻ ligand. (e) TEM images of S²⁻-DDA⁺ capped CdSnanorods. (f) Absorption spectra of 5.5 nm CdSe NCs capped withdifferent ligands.

FIG. 1C shows FTIR spectra of 5.5 nm CdSe NCs taken before and after theexchange of organic ligands with S²⁻. The transfer of NCs from tolueneto FA resulted in complete disappearance of the bands at 2852 and 2925cm⁻¹ corresponding to C—H stretching in the original organic ligands.Other bands in the FTIR spectra for S²⁻ capped NCs can be assigned tothe solvent and (NH₄)₂S (FIG. 8). For example, FTIR spectra of S²⁻cappedCdSe NCs prepared from water dispersion showed no observable C—H bands.These results confirm the efficacy of S²⁻ ligands in complete removal ofthe original organic ligand, forming all-inorganic colloidal NCs.

NC surface with electrophilic sites, for example, a Cd rich surface ofCdSe NC, should favor adsorption of nucleophilic S²⁻ ions compared topositively charged counter ions (e.g., K⁺ or NH₄ ⁺ ions). In addition,in polar solvents cations are generally more solvated than anions.Colloidal stabilization of NCs in FA was achieved through bindingnegatively charged S²⁻ ions to NC surface, leading to the formation ofan electrical double layer around each NC. The negative charging of CdSeNC surface resulted in a negative (−40 mV) ζ-potential, sufficient forelectrostatic stabilization of the colloidal dispersion. Dynamic lightscattering (DLS) studies also confirmed single-particle populations inthe solutions of S²⁻ capped CdSe NCs (FIG. 1D, for NCs with the TEMimage shown in the inset). The direct comparison of the volumedistribution curves calculated from DLS revealed that averagehydrodynamic diameter of S²⁻ capped NCs was smaller by ˜1.7 nm than thatof the same NCs capped with n-tetradecylphosphonic acid, which accountedfor the effective length of the hydrocarbon chains.

In polar solvents like FA, we used small inorganic cations like K⁺, Na⁺,NH₄ ⁺ to balance the negative charge of anions adsorbed on the NCsurface. These cations can be replaced with hydrophobic quaternaryammonium ions by treatment with didodecyldimethylammonium bromide(DDA⁺Br⁻), rendering NC insoluble in FA, but soluble in toluene. Thistechnique was applied to S²⁻ capped CdS nanorods. FIG. 1E shows a TEMimage of S²⁻-DDA⁺ capped CdS nanorods. The shape and dimensions ofnanorods remained intact during the transfer from toluene (ODPA-TOPOligands) to FA (S²⁻ ligands) to toluene (S²⁻-DDA⁺ ion pair ligands).

Stable colloidal solutions of CdSe NCs can be prepared in FA usingdifferent chalcogenide (S²⁻, Se²⁻ and Te²⁻) and hydrochalcogenide (HS⁻,HSe⁻ and HTe⁻) ions. FIG. 1F shows the absorption spectra for colloidalsolutions of the same CdSe NCs stabilized with different ligands. In allcases of chalcogenide and hydrochalcogenide ligands the formation ofstable colloidal solutions of negatively charged NCs was observed. Itshould be noted that selenide and telluride ions can be easily oxidizedin air and corresponding colloidal solutions should be handled underinert atmosphere.

Chalcogenide ions can stabilize different nanomaterials in polarsolvents. For example, FIG. 2A shows S²⁻ capped CdSe/ZnS core-shell NCswith strong band edge PL with QE about 25%. All-inorganic films of S²⁻capped CdSe/ZnS NCs also showed bright emission, even after annealing at250° C. for 30 min. S²⁻ ligand worked well for InP NCs (FIG. 2B)providing the first example of all-inorganic colloidal III-V NCs. Theabsorption spectra of S²⁻ capped InP NCs in FA were similar to those ofthe organically capped NCs in toluene. The ζ-potential of S²⁻ capped InPNCs was about −60 mV. The ability of chalcogenide ions to providecolloidal stabilization was not limited to semiconductors; FIG. 2Ccompares the absorption spectra for spherical 7 nm Au NCs capped withdodecanethiol in toluene and with S²⁻ ions in FA. A 11 nm red-shift forsurface plasmon band in S²⁻ capped Au NCs was observed, consistent withthe difference in dielectric constants of toluene and FA. DLS studiesrevealed that S²⁻ capped Au colloids retained size distribution afterthe ligands exchange (FIG. 2D), with no agglomeration for both organicand S²⁻ capped NCs. The ζ-potential of S²⁻ capped Au NCs was about −60mV.

Regarding FIGS. 2A-2D; (a) Absorption and PL spectra of CdSe/ZnScore/shell NCs capped with S²⁻ ligands dispersed in FA; inset shows theluminescence of NC dispersion upon illumination with 365 nm UV light.(b) Absorption spectra of InP NCs capped with organic ligands and S²⁻ligands dispersed in toluene and FA respectively. Inset shows theoptical photograph of colloidal dispersion of InP NCs in FA. (c)Absorption spectra of Au NCs capped with dodecanethiol and S²⁻ ligandsdispersed in toluene and FA, respectively. Inset shows the opticalphotograph of a colloidal dispersion of Au NCs in FA. (d) DLSsize-distribution plots for Au NCs capped with the organic and S²⁻ligands.

Mild oxidation of chalcogenide ions typically results in the formationof polychalcogenide ions (S_(n) ²⁻, Se_(n) ²⁻, Te_(n) ²⁻). In fact,polychalcogenides are often present as an impurity in commercial metalchalcogenide chemicals. As a simple test, aqueous sulfide solutionsshould be colorless whereas polysulfides absorb visible light and appearyellow to red. ESI-MS was used to identify the presence and stability ofpolychalcogenides in different solvents. The mass spectra of the redNa₂Sn solution prepared by adding an excess of sulfur to aqueous 0.7 MNa₂S solution showed the presence of polysulfides with n=5, 6, and 7unlike to colorless Na₂S solution (FIGS. 9-11). Both (NH₄)₂S_(n) andNa₂Sn solutions can stabilize colloidal NCs in polar solvents,suggesting that polysulfides behave similarly to sulfide ions.

Mixed Chalcogenide TeS₃ ²⁻ Ligands.

Having molecular structure intermediate between MCCs andpolychalcogenide ions, mixed chalcogenide ions such as TeS₃ ²⁻ representanother interesting choice of metal-free ligands. For the study,light-yellow aqueous (NH₄)₂TeS₃ was used to stabilize colloidal CdSe NCsvia ligand exchange in FA. ESI-MS spectra show that (NH₄)₂TeS₃ aqueoussolution contains mainly TeS₃ ²⁻ ions along with Te₂S₅ ²⁻ (FIG. 3A).Adsorption of submonolayer amounts of Te—S ions onto the surface of 4.4nm CdSe NCs was confirmed by elemental analysis, indicating Te-to-Cdatomic ratio 0.25. The atomic ratio of S-to-Te equal to 3.67 suggeststhe adsorption of TeS₃ ²⁻ ions, along with possibility for someadditional S²⁻ incorporated onto the NC surface. Note that Cd-to-Seratio of 1.25 was similar to that in original NCs. The integrity of CdSeNC core was also evident from the similar excitonic absorption featuresin the optical absorption spectra (FIG. 3B). The main difference lies inthe high energy part of absorption spectra (5450 nm) showing thecontribution from the TeS₃ ²⁻ ions. Surface affinity of TeS₃ ²⁻ ions israther similar to that of MCC ligands. For example, stable colloidalsolutions of PbS NCs can be formed with TeS₃ ²⁻ ions in FA, whereascapping with S²⁻ cannot provide stable colloidal solutions for IV-VINCs.

The presence of two chalcogens in different oxidation states opensinteresting possibilities for surface redox reactions. Te⁴⁺ can oxidizeits own ligand (S⁻²) releasing elemental Te and S, which explainslimited stability of (NH₄)₂TeS₃ in the solid state, under light or heat.Another redox pathway is the reaction of Te⁴⁺ with the NC material. Forexample, metal telluride NCs such as CdTe and PbTe slowly reacted withTeS₃ ²⁻ releasing elemental Te (Te⁴⁺+Te²⁻→Te⁰ comproportionationreaction) and leading to partial or complete conversion to CdS and PbS.For PbTe NCs this reaction occurred within minutes after the ligandexchange, while more covalent CdTe converted to CdS only upon heating ofNC films at 120° C. CdSe NCs are significantly more stable, and onlypartially react under the thermal treatment (FIG. 12). At the same time,PbS NCs do not react with TeS₃ ²⁻ as confirmed by powder XRD patternsfor dried and annealed samples (FIG. 13). Therefore, the use of mixedchalcogenide ligands can be advantageous for compositional modulationsin nanogranular solid materials such as thermoelectric Pb and Sbchalcogenides.

Regarding FIGS. 3A and 3B; (a) ESI-MS spectrum of (NH₄)₂TeS₃ solution.The inset compares an experimental high-resolution mass-spectrum withsimulated spectra for TeS₃H⁻ and TeS₃ ions. (b) Absorption spectra for(NH₄)₂TeS₃, for CdSe NCs capped with the organic ligands in toluene andfor CdSe NCs capped with TeS₃ ²⁻ ions in formamide.

Other Metal-Free Anionic Ligands (OH⁻- and NH₂).

Stabilization of colloidal NCs with other inorganic anions was explored.OH⁻ and NH₂ ⁻ successfully replaced organic ligands at the surface ofCdSe NCs and stabilized colloidal solutions in FA. FIG. 4A shows thatabsorption and PL spectra of OH⁻ and NH₂ ⁻ capped 5.5 nm CdSe NCs didnot change compared to original NCs (FIG. 1Bb); no change was alsodetected in the XRD patterns of corresponding NCs (FIG. 14). Similar toS²⁻ capped NCs, OH⁻ capped CdSe NCs exhibited negative ζ-potentials inpolar solvents, consistent with the electrostatic stabilization. Thevalue of ζ-potential for OH⁻ capped CdSe NCs in FA (−20 mV) wassubstantially lower than that for S²⁻ capped NCs. Right after the ligandexchange, colloidal solutions can be easily filtered through a 0.2 μmfilter but they slowly aggregated and precipitated in several days.Unlike S²⁻ capped NCs, FTIR spectra of OH⁻ capped CdSe NCs (FIG. 15) didnot show complete removal of CH₂ bands at 2852 and 2925 cm⁻¹, but theintensity of C—H bands decreased by >90% compared to the original NCs.Similar behavior and ζ-potentials have been observed for NH₂ capped CdSeNCs. These observations point to somewhat lower affinity of OH⁻ and NH₂⁻ toward CdSe surface compared to chalcogenides. In contrast, ZnSe NCsdid not form stable colloidal solutions with S²⁻ ligands but can beeasily stabilized with OH⁻ in FA (FIG. 4B). The possible origin of suchselectivity will be discussed in the next section.

Regarding FIGS. 4A and 4B; (a) Absorption and PL spectra of OH⁻ and NH₂⁻ capped CdSe NCs dispersed FA. Inset shows a photograph of thecolloidal solution. (b) Absorption spectra of ZnSe NCs capped witholeylamne and OH⁻ ligands dispersed in toluene and FA, respectively.

Application of Hard and Soft Acid and Base (HSAB) Classification toColloidal Nanostructures.

In 1963 Pearson proposed HSAB principle that explained many trends instability of the complexes between Lewis acids and bases by classifyingthem into “hard” and “soft” categories. Generally, soft acids formstable complexes with soft bases, whereas hard acids prefer hard bases.The inorganic ligands used in this work represent classical examples ofsoft (highly polarizable HS⁻, S²⁻, Se²⁻) and hard (less polarizable OH⁻,NH₂) Lewis bases. It was noticed that original ligands on Au and CdTeNCs can be easily replaced with S²⁻, Se²⁻ or HS⁻ ligands, but did notform stable colloidal solutions in the presence of OH⁻. In contrast,ZnSe (FIG. 4b ) and ZnO NCs can be stabilized with OH⁻ but did notinteract with S²⁻ and HS⁻. Other NCs, including CdSe and CdS, formedstable colloidal solutions with both hard and soft ligands. Thisdifference in behavior was attributed to the differences in chemicalaffinity between NCs and Lewis bases and proposed that HSAB theory,originally developed for molecular coordination compounds, can beextended to the world of nanomaterials (FIG. 72). The ligands used inthis work bind primarily to the electrophilic sites at NC surface. Inagreement with HSAB, soft Au⁰ and Cd²⁺ sites exhibited stronger affinityto soft S²⁻ and HS⁻ ligands compared to hard OH⁻, whereas harder Zn²⁺sites in ZnSe and ZnO NCs had higher affinity to OH⁻ rather than to HS⁻.More quantitative information on the binding affinity can be obtainedfrom the comparison of ζ-potentials related to the charges on NC surface(FIG. 16). The extension of HSAB principle to NCs appears to fail forInP and InAs NCs. Free In is a hard acid, while InAs and InP behave assoft acids preferentially binding to S²⁻ and HS⁻ rather than to OH⁻.This discrepancy, however, can be explained by taking into account thecharacter of chemical bonding inside NCs. Small difference inelectronegativity between In and P or As led to very small positiveeffective charge on the In surface sites, much smaller than the chargeon isolated In ions. Low charge density softens the acidic sites at InPand InAs NCs surface. To further test this hypothesis, In₂O₃ NCs weresynthesized, where the bonding has a higher ionic character compared toInP and InAs, and their interaction with OH⁻ and HS⁻ ligands wasstudied. Both HS⁻ and OH⁻ bound to In₂O₃ NC surface resulting in similarζ-potentials (−40 mV), suggesting that the metal surface sites in In₂O₃NCs were harder than those in InP and InAs NCs.

FIG. 72 shows (a) typical metal complexes and (b) a nanocrystal withsurface ligands. Similarities between metal-ligand (M-L) bonding in bothclasses of compounds should be noted. HSAB theory, originally developedfor metal complexes, can also apply to colloidal nanomaterials.

Comparison of Metal-Free Inorganic Ligands to MCCs.

Negatively charged metal-free ligands can be viewed as an addition tothe family of all-inorganic colloidal NCs that started with thediscovery of MCC ligands. In this section, observations comparingmetal-free inorganic ligands to MCCs are summarized.

The solutions of MCCs in polar solvent are expected to contain a certainequilibrium concentration of free chalcogenide ions. The possibility ofusing chalcogenide ions to prepare stable colloidal solutions of NCs inhydrazine, typically used with MCC ligands have been explored. It turnedout that chalcogenides can not stabilize II-VI, IV-VI and III-V NCs inhydrazine. Probably, the dielectric constant of hydrazine (ε=51) and itssolvation ability were not sufficient to obtain stable colloids withpure chalcogenides. In contrast, Sn₂Se₆ ⁴⁻ and other MCC ligandsprovided long-term, up to several years, stability to CdSe NC colloidsin hydrazine. In FA, both chalcogenides and MCCs generated stablecolloidal solutions, e.g., for Au and CdSe NCs. Direct comparisonrevealed that CdSe NCs capped with SnS₄ ⁴⁻ or AsS₃ ³⁻ MCC ligandsexhibited higher ζ-potentials, typically in the range of −50 to −80 mV.That probably corresponded to a higher affinity of MCCs toward CdSe NCs.On the other hand, S²⁻ was superior for CdSe/ZnS NCs, for which manyMCCs showed poor affinity. MCCs and mixed chalcogenide ligands werebetter for Pb chalcogenide NCs, for which S²⁻-capping failed. Comparedto the MCCs, free chalcogenide ions were somewhat more prone tooxidative degradation. Both, sulfide-based MCC-capped NCs and S²⁻-cappednanomaterials can be prepared and handled in air, but S²⁻-capped NCswere stable much longer if prepared and stored in a glovebox.

Electrostatic Stabilization of Colloidal Nanocrystals in the Absence ofCharged Ligands (“Ligand-Free” Nanocrystals).

Inorganic ligands that provided negative charge to the NC surface havebeen discussed. All-inorganic positively charged NCs are rarer,presumably, without wishing to be bound by theory, because NCs typicallyhave electrophilic metal-rich surfaces with preferential affinity towardnucleophilic ligands. Recently, Murray and coworkers reported colloidalNCs with positively charged surface. They used nitrosoniumtetrafluoroborate (NOBF₄) and aryldiazonium tetrafluoroborate to performligand exchange on Fe₃O₄, FePt, Bi₂S₃ and NaYF₄ NCs, resulting inpositively charged NC surface. Unfortunately this approach did not workwith CdSe and other semiconductor NCs. Nitrosonium salts are known asstrong one-electron oxidants, resulting in the etching of NC surfaceimmediately after the addition of NOBF₄ (FIG. 17).

To cleave the bonds between CdSe NC and carboxylate organic ligandswithout oxidizing the NCs, HBF₄ and HPF₆ in polar solvents were used.This reaction resulted in the phase transfer of NCs from non-polarsolvent (toluene) to a polar solvent (FA). In the case of oleylamine andoleate capping ligands, H⁺ cleaved the Cd—N and Cd—O bonds, asschematically shown in FIG. 5A, leaving behind positively charged metalsites at the NC surface. HBF₄ treatment was efficient for NCs cappedwith carboxylate and alkylamine ligands whereas CdSe NCs capped withalkylphosphonic acid remained in the toluene phase even after extendedHBF₄ treatment. BF₄ ⁻ and PF₆ ⁻ are known as very weak nucleophiles.These weakly coordinating anions did not bind to the NC surface, insteadthey acted as counter ions in the electrical double layer around NCs.FIG. 5B shows the absorption and PL spectra of 5.5 nm CdSe NCs andCdSe/ZnS core-shells after treatment with HBF₄ in FA. The size,size-distribution and band-edge emission remained, however, with asignificant drop in the PL efficiency (<0.5% for CdSe NCs and ˜2% forCdSe/ZnS NCs). ζ-potentials of CdSe and CdSe/ZnS NCs in FA were measuredto be around +30 mV. Similar values were obtained with HPF₆. The removalof organic ligands was not possible with NaBF₄ and NaPF₆, emphasizingthe role of H⁺ ions in the reaction, which was also suggested inprevious reports. FIG. 5C compares FTIR spectra of CdSe NCs capped withorganic ligands and FTIR spectra of the same CdSe NCs after HBF₄treatment. The absence of C—H bands at 2852 and 2925 cm⁻¹ constitutedcomplete removal of the original organic ligand. The band centered at2890 cm⁻¹ was from FA (FIG. 8). Apart from the FA related bands, a newsharp band at 1083 cm⁻¹ corresponding to BF₄ ions was observed.

Regarding FIGS. 5A-5C; (a) Removal of the organic ligands from thesurface of CdSe NCs by HBF₄ treatment. For simplicity, the monodentatebinding mode of carboxylate group is shown. The proposed mechanism canhold for other chelating and bridging modes as well. (b) Absorption andPL spectra of ligand-free colloidal CdSe and CdSe/ZnS NCs obtained afterHBF₄ treatment. (c) FTIR spectra of organically capped and ligand-freeCdSe NCs after HBF₄ treatment.

Growth of Metal Calcogenides on NCs at Room Temperature.

For the growth of metal chalcogenides on nanocrystals at roomtemperature, several approaches have been studied either in polarsolvent or non polar solvent. In 2011, Talapin and co-workers havereported the use of chalcogenides and hydrochalcogenides as cappingligands for nanocrystals in polar solvents such as formamide ofN-methylformamide. These ligands create a bond with the surface ofnanocrystals and allow them to be soluble in a polar solvent. Hence itcan be considered as the addition of a half monolayer, the second halfmonolayer would be induced through the addition of cadmium either inpolar phase or in non polar phase. In an exemplary synthetic approach,ammonium sulfide or potassium sulfide are chosen as the sulfide sourceand cadmium nitrate, cadmium acetate, cadmium perchlorate or cadmiumoleate are used as the cadmium source.

Several methods are contemplated, comprising use of both a polar and anonpolar solvent. Contemplated nonpolar solvents include toluene,chloroform, methylene chloride, hexanes, octadecane, and mixturesthereof. Contemplated polar solvents include FA, DMF, NMFA, methanol,ethanol, and mixtures thereof. In some cases, the method furthercomprises adding a quaternary ammonium salt. Contemplated quaternaryammonium salts include Tetramethylammonium bromide; Tetramethylammoniumchloride; Tetramethylammonium hexafluorophosphate; Tetramethylammoniumhydroxide pentahydrate; Tetramethylammonium hydroxide;Tetramethylammonium hydroxide; Tetramethylammonium iodide;Tetramethylammonium nitrate; Tetramethylammonium perchlorate;Tetramethylammonium tetrafluoroborate; Triethylmethylammonium chloride;Tetraethylammonium bromide; Tetraethylammonium chloride monohydrate;Tetraethylammonium hydroxide; Tetraethylammonium hydroxide;Tetraethylammonium hydroxide; Tetraethylammonium iodide;Tetraethylammonium nitrate; Tetraethylammonium perchlorate;Tetraethylammonium tetrafluoroborate; Tetraethylammoniump-toluenesulfonate; (1-Hexyl)trimethylammonium bromide;Phenyltrimethylammonium bromide; Phenyltrimethylammonium chloride;Phenyltrimethylammonium iodide; Phenyltrimethylammonium methosulfate;Benzyltrimethylammonium bromide; Benzyltrimethylammonium chloride;Benzyltrimethylammonium hexafluorophosphate; Benzyltrimethylammoniumhydroxide; Benzyltrimethylammonium hydroxide; Benzyltrimethylammoniumiodide; (1-Butyl)triethylammonium bromide; (1-Octyl)trimethylammoniumbromide; Tetra-n-propylammonium bromide; Tetra-n-propylammoniumchloride; Tetra-n-propylammonium hydrogen sulfate;Tetra-n-propylammonium hydroxide; Tetra-n-propylammonium iodide;Phenyltriethylammonium iodide; Methyltri-n-butylammonium bromide;Methyltri-n-butylammonium chloride; (1-Decyl)trimethylammonium bromide;Benzyltriethylammonium bromide; Benzyltriethylammonium chloride;Benzyltriethylammonium hydroxide; Benzyltriethylammoniumtetrafluoroborate; (1-Dodecyl)trimethylammonium chloride;(1-Dodecyl)trimethylammonium bromide; Benzyltri-n-propylammoniumchloride; Tetra-n-butylammonium acetate; Tetra-n-butylammonium acetate,Tetra-n-butylammonium bromide; Tetra-n-butylammonium chloride;Tetra-n-butylammonium chloride; Tetra-n-butylammoniumhexafluoro-phosphate; Tetra-n-butylammonium hydrogen sulfate;Tetra-n-butylammonium hydroxide; Tetra-n-butylammonium hydroxide;Tetra-n-butylammonium hydroxide; Tetra-n-butylammonium hydroxide;Tetra-n-butylammonium iodide; Tetra-n-butylammonium nitrate;Tetra-n-butylammonium perchlorate; Tetra-n-butylammonium perchlorate;Tetra-n-butylammonium phosphate; Tetra-n-butylammonium sulfate;Tetra-n-butylammoniumtrifluoromethane-sulfate;(1-Tetradecyl)trimethylammonium bromide; (1-Tetradecyl)trimethylammoniumchloride; (1-Hexadecyl)trimethylammonium bromide;Ethyl(1-hexadecyl)dimethylammonium; Tetra-n-pentylammonium iodide;Benzyltri-n-butylammonium bromide; Benzyltri-n-butylammonium chloride;Benzyltri-n-butylammonium iodide; (1-Hexadecyl)pyridinium bromidemonohydrate; (1-Hexadecyl)pyridinium chloride monohydrate;Di-n-decyldimethylammonium bromide; Tetra-n-hexylammonium bromide;Tetra-n-hexylammonium hydrogen sulfate; Tetra-n-hexylammonium iodide;Tetra-n-hexylammonium perchlorate; Di-n-dodecyldimethylammonium bromide;Tetra-n-heptylammonium bromide; Tetra-n-heptylammonium iodide;Tetra-n-octylammonium bromide; Dimethyldistearylammonium chloride;Tetra-n-dodecylammonium iodide; Tetraoctadecylammonium bromide, ormixtures thereof.

In some cases, the method further comprises adding an alkyl amine to theadmixture of nanoparticle and inorganic capping agent. Contemplatedalkyl amines include mono-, di- and tri-substituted alkyl amines. Thealkyl group can be a hydrocarbon of 6 to 20 carbon, optionally with oneor more double bonds. A specific alkyl amine contemplated is oleylamine.

In one method, in a vial, 1 ml of FA, 30 μl of a solution at 0.1M of K₂Sin FA ((NH₄)₂S can also be used as sulfide precursor), 1 ml of toluene,60 μl of a solution at 0.1M of DDAB (didodecyldimethylammonium bromide)in toluene and 200 μl of NCs (20 mg/ml) are stirred for five minutes.There is no transfer of phase and the NCs are stabilized with S-DDA intoluene. The non polar phase is washed two times with 1 ml of pureformamide by stirring one minute each time. Then 30 μl of a solution at0.1M of cadmium oleate, Cd(OA)₂, in toluene is added to the mixture andstirred for one minute. Finally the NCs are precipitated with ethanoland resuspended in 400 μl of toluene. By repeating the same process withincreasing the amount of precursors, it is possible to grow at least 10monolayers.

It is possible to prevent from the precipitation of nanocrystals byusing octadecene (ODE) as a solvent for NCs and by extraction the excessof Cd(OA)₂ with addition of toluene (or hexane or chloroform) andmethanol (or ethanol). The NCs will stay in ODE while the excess ofprecursor will go in the mixture of Methanol/Toluene.

In another method, in a vial, 1 ml of FA, 2 μl of (NH₄)₂S, 200 μl of NCs(20 mg/ml), 1 ml of toluene and 15 μl of oleylamine are stirred for fiveminutes. There is no transfer of phase since NCs are stabilized byoleylamine. The non polar phase containing the NCs, is washed two timeswith pure formamide. Then 1 ml of formamide, and 30 μl of a solution at0.1M of Cd(OAc)₂ in formamide are added to the polar phase and stirredfor 5 minutes. The nanocrystals are still stabilized by oleylamine inthe non polar phase, but cadmium has grown on the sulfide on surface ofnanocrystals. The non polar phase is rinsed two times with formamide.The successive monolayers are grown by the same way. This processdoesn't require a precipitating step after each monolayer.

Growth by Phase Transfer:

In a vial 1 ml of FA, 2 μl of (NH₄)₂S, 200 μl of NCs (20 mg/ml) and 1 mlof toluene are stirred until complete phase transfer. The NCs are thenmoved back with addition of toluene, DDAB and Et₄NBr in formamide. Thetransfer from polar to non polar phase is fast. The addition of Et₄NBrallows an increase of the ionic strength. The non polar phase is thenrinse two times with fresh formamide. Then 1 ml of formamide, and 30 μlof a solution at 0.1M of Cd(OAc)₂ in formamide are added to the polarphase and stirred for 5 minutes. The polar phase is then rinse two timeswith fresh formamide.

Growth in Polar Phase:

In a vial 1 ml of NMFA, 2 μl of (NH₄)₂S, 200 μl of NCs and 1 ml ofhexane are stirred until complete phase transfer. The NCs are thenrinsed two times with hexane, precipitated with acetonitrile andredispersed with NMFA. Then 301l of a solution at 0.1M of Cd(OAc)2 inNMFA is added and stirred for 30 seconds. The NCs are then precipitatedwith toluene and redispersed with NMFA.

Colloidal NCs with Metal-Free Inorganic Ligands for Device Applications.

Small inorganic ligands like S²⁻ or Se²⁻ do not introduce insulatingbarriers around NCs and provide an exciting opportunity for theintegration of colloidal NCs in electronic and optoelectronic devices.Detailed investigations of NCs with these new capping ligands will be asubject of a separated study. Here results on electron transport in thearrays of S²⁻ capped CdSe NCs as well as CdSe/CdS and CdSe/ZnScore-shell NCs are shown.

The electron mobility in NC arrays can be conveniently measured fromtransfer characteristics of a field-effect transistor. For thesestudies, 20-40 nm thick close packed NC films were deposited on highlydoped Si wafers with 100 nm thick SiO₂ thermal gate oxide. The NC filmswere spin-coated from FA solutions at elevated temperatures (80° C.),followed by short annealing at 200-250° C. When (NH₄)₂S was used tostabilize the solution of CdSe NCs, this mild thermal treatment resultedin the complete removal of surface ligands and led to much highercarrier mobility in the NC array. Source and drain Al electrodes werepatterned on the annealed NC films using a shadow mask. FIGS. 6A-6C showsets of drain current (I_(D)) vs. drain-source voltage (V_(DS)) scans atdifferent gate voltages (V_(G)) for devices made of (NH₄)₂S capped 4.6nm CdSe NCs (originally capped with oleylamine and oleic acid), 10 nmCdSe/CdS, and 8 nm CdSe/ZnS core/shell NCs. I_(D) increased withincreasing V_(G), characteristic of the n-type transport.

The electron mobility corresponding to the linear regime of FEToperation (measured at V_(DS)=2V across a 80 μm long channel) for a filmof CdSe NCs capped with (NH₂)₂S annealed at 200° C. was μ_(lin)=0.4cm²V⁻¹s⁻¹ with I_(ON)/I_(OFF)˜10³ (FIGS. 6a, d ). FETs assembled fromCdSe/CdS (FIGS. 6B, 6E) and CdSe/ZnS (FIGS. 6C, 6F) core-shell NCscapped with (NH₄)₂S ligands annealed at 250° C. showed μ_(lin)=0.04cm²V⁻¹s⁻¹ with I_(ON)/I_(OFF)˜10⁴ at V_(DS)=2V and μ_(lin)=6·10⁻⁵cm²V⁻¹s⁻¹ with I_(ON)/I_(OFF)˜10² at V_(DS)=4V, respectively. Theelectron mobility measured in the saturation mode of FET operation(V_(DS)=30V, FIG. 18) was slightly higher than μ_(lin) for CdSe andCdSe/ZnS NCs: 0.5 cm²V⁻¹s⁻¹ and 9·10⁻⁵ cm²V⁻¹s⁻¹, correspondingly. Thedevices assembled from CdSe/CdS core-shell NCs showed similar electronmobility in both linear and saturation regimes.

The channel currents were orders magnitude lower for CdSe NCs cappedwith K₂S ligands as compared to (NH₄)₂S ligands. Without wishing to bebound by theory, low conductivity is attributed to the presence of K⁺ions creating local electrostatic barriers around NCs and behaving asthe electron scattering centers. The devices assembled from CdSe NCscapped with (NH₄)₂TeS₃ also showed n-type transport with μ_(lin)=0.01cm²V⁻¹s⁻¹ (FIG. 19).

Although the films of CdSe/ZnS core-shell NCs showed rather low carriermobility, these NCs preserved bright band-edge photoluminescence evenafter annealing at 250° C. (inset to FIG. 7C). Generally, metal-freeligands allowed obtaining conductive NC layers without introducingforeign metal ions that often introduce recombination centers. As aresult, efficient band-edge PL in electronically conductive NC arrayswere observed for the first time (FIG. 6C).

To conclude this work, it has been demonstrated that various metal-freeinorganic ligands like S²⁻, OH⁻ and NH₂ ⁻ can behave as the cappingligands for colloidal semiconductor and metallic NCs. These ligandssignificantly enrich the colloidal chemistry of all-inorganic NCs, andrepresent a useful strategy for integration of colloidal nanostructuresin electronic and optoelectronic devices.

Regarding FIGS. 6A-6C; (a-c) Plots of drain current I_(D) vs.drain-source voltage V_(DS), measured as different gate voltages V_(G)for the field-effect transistors (FETs) assembled from colloidal NCscapped with (NH₄)₂S: (a) CdSe, (b) CdSe/CdS core-shells and (c) CdSe/ZnScore-shells. Insets to panel (c) show the photoluminescence spectrum(left) and a photograph (right) for CdSe/ZnS NC film capped with (NH₄)₂Sligands annealed at 250° C. for 30 min. (d,e) Plots of I_(D) vs. V_(G)at constant V_(DS)=2V used to calculate current modulation and linearregime field-effect mobility for FETs using CdSe and CdSe/CdS NCs. (f)Plots of I_(D) vs. V_(G) measured at constant V_(DS)=4V in a FETassembled from CdSe/ZnS NCs. L=80 μm, W=1500 μm, 100 nm SiO₂ gate oxide.

Mobility Calculations.

Linear Range Mobility (μ_(lin));

At low V_(DS) the current I_(D) increases linearly with V_(DS). In thismode the FET operates like a variable resistor and the FET is said to beoperating in a linear regime. I_(D) can be obtained from the followingequation:

$\begin{matrix}{{I_{D} = {\frac{{WC}_{i}\mu_{lin}}{L}\left( {V_{G} - V_{T} - \frac{V_{D}}{2}} \right)V_{D}}},} & ({S1})\end{matrix}$where L is the channel length, W is the channel width, C_(i) is thecapacitance per unit area of the dielectric layer, V_(T) is thethreshold voltage, and μ_(lin) is the linear regime field-effectmobility. μ_(lin) is usually calculated from the transconductance(g_(m)) by plotting I_(D) versus V_(G) at a constant low V_(D). Theslope of this plot is equal to g_(m):

$\begin{matrix}{{{g_{m} = \frac{\partial I_{D}}{\partial V_{G}}}}_{V_{D} = {const}} = {\frac{{WC}_{i}V_{D}}{L}\mu_{lin}}} & ({S2})\end{matrix}$Saturation Mobility (μ_(sat));At high V_(DS) the current saturates as the channel “pinches off” nearthe drain electrode. If V_(DS) voltage is increased further, thepinch-off point of the channel begins to move away from the draintowards the source. The FET is said to be in the saturation regime. ForV_(DS)≥(V_(GS)−V_(T)), I_(D) can be expressed as:

$\begin{matrix}{I_{D} = {\frac{{WC}_{i}\mu_{sat}}{2L}\left( {V_{G} - V_{T}} \right)^{2}}} & ({S3})\end{matrix}$where μ_(sat) is the saturation regime field-effect mobility. Thisparameter is typically calculated from the slope of [I_(D)]^(1/2) vs.V_(G) curve.

Hysteresis-Free Operation of Field-Effect Transistors on CdSeNanocrystals Capped with S²⁻ Ligands

For many practical applications of nanocrystal transistors, solar cellsand other electronic and optoelectronic devices, the hysteresis freeoperation is a very important requirement that can be satisfied inaddition to the high field effect mobility. Hysteretic response in FETsappears as a difference in the source-drain current (I_(DS)) valuesobserved during forward and backward sweeps of the gate voltage.(V_(GS)) The mechanisms that cause hysteresis in transistors have beenextensively studied on bulk inorganic or organic field effecttransistors. In general, i) interface-trapped charge, ii) fixed chargein the dielectric layer and iii) mobile ions in the dielectric layer areknown to be responsible for the hysteresis. By preventing thesemechanisms, hysteresis free transistors have been reported in thosefields.

However, the previous reports on nanocrystal transistors always haveshown large hysteresis both in the output and transfer characteristics.Considering above mentioned mechanisms, the previously reportedhysteresis in nanocrystal transistors can originate from hydrophilicSiO₂ surface, of which hydroxyl group can trap electron, and chargetrapping ligand surrounding the nanoparticles. For example, by using anamorphous fluoropolymer (Cytop™) (hydroxyl free polymer) as thedielectric layer and In₂Se₄ ²⁻ capped CdSe nanocrystals as thesemiconductor layer, one can dramatically reduce the hysteresis oftransfer curve. However one can still observe a certain hysteresis whichcan originate from the hole trapping by In₂Se₄ ²⁻ ligands.

Based on the assumption that S²⁻ ligands trap holes less efficientlythan molecular metal chalcogenide (MCC) ligands, transistors werefabricated with top-gate geometry using Cytop™ as the gate dielectriclayer and S²⁻ capped CdSe as the semiconductor layer. As mentionedearlier, the use of Cytop™ can guarantee hydroxyl group free interfacesbetween semiconductor and dielectric layer, thus enabling electrontrap-free operation for transfer curves. The devices were fabricated asfollows: MCC- or S2-capped nanocrystal solutions were spin-coated ontoheavily doped Si wafers served as substrate under dry nitrogenatmosphere. As-deposited NC films were annealed at 200° C. for 30 minand Al source-drain electrode was patterned by thermal evaporation withchannel length of 150 μm and channel width of 1500 μm. For theinsulating layer, Cytop CTL-809M (solvent: CT-Solv.180) from AsahiGlass, Japan was spin coated onto the nanocrystal layer and dried for 30min. at 150° C. The thickness of the insulating layer was determined tobe 800-850 nm thick, which gives a gate capacitance of C_(i)=2.2-2.3nF/cm². Finally Al gate electrode was deposited by thermal evaporation.(FIG. 20)

FIG. 21 shows very ideal transfer and output characteristics of S²⁻capped CdSe transistors without any hysteresis which tell that theinterface between S²⁻ capped CdSe nanocrystal and Cytop™ is free fromboth electron and hole traps. Considering that the transistor based onIn₂Se₄ ²⁻ capped CdSe nanocrystals using same transistor geometry stillshowed some hysteresis, it is probable that S²⁻ capped CdSe nanocrystalshave lower hole trap density than MCC capped CdSe nanocrystals. Theseresults further confirm that S²⁻capped colloidal nanocrystal transistorscan be useful for real application in integrated circuit with highreliability.

Producing Chalcogenide-Capped III-V Semiconductors

Bridging InAs NCs with (N₂H₄)(N₂H₅)Cu₇S₄ MCC ligands can lead to veryhigh electron mobility approaching 15 cm²/Vs. In addition, bipolar(positive/negative) photoresponse of MCC-capped InAs NCs solids wasobserved that can be tuned with ligands chemistry, temperature andwavelength and the doping of NC solid.

III-V semiconductors such as GaAs, InP, InAs combine a direct band gapwith very high mobility, reliable p- and n-type doping and othercharacteristics making them excellent materials for various electronicand photonic applications. The fastest commercial transistors and themost efficient solar cells employ III-V semiconductors. At the sametime, despite all the benefits, various technological difficulties withgrowing and processing single crystals do not allow III-V materials tosuccessfully compete with silicon and chalcogenides for large consumermarkets. As a possible solution, colloidal nanocrystals such as InAs andInP, can be used as a cost-efficient alternative to III-V singlecrystals for applications, for example, in photovoltaics, lightdetectors, field effect transistors (FET), and light emitting diodes(LED). For these applications efficient charge transport between NCs ispreferred. The electronic properties of functional materials made ofsemiconductor NCs strongly depend on the ability of charge carriers tomove from one NC to another through the interparticle boundaries. Chargetransport through interfaces and grain boundaries is also of generalimportance for various applications of granular semiconductingmaterials.

Colloidal synthesis of high quality semiconductor nanocrystals hasrequired the use of organic surface ligands that form insulating shellsaround each NC and negatively affect the charge transport. Removal oforganic surfactants via thermal or chemical treatment often leads tosurface traps and NCs sintering. A more useful approach is to chemicallytreat NCs with small ligands. These techniques have been originallydeveloped for II-VI (e.g., CdSe) and IV-VI (e.g., PbSe) NCs. A fewreported charge transport studies for InAs NCs used eitherpostdeposition ligand exchange treatment of InAs NCs with ethanedithiolor solution ligand exchange of TOP capped InAs with aniline followed bypostdeposition crosslinking with ethylenediamine (EDA). Such treatmentsconverted highly insulating organics-capped InAs NCs into semiconductingNC solids.

Ligand exchange of organic capped nanocrystals with MCCs. All ligandexchange procedures were performed inside a nitrogen-filled glovebox(sub-1 ppm O₂ and H₂O levels) using anhydrous solvents. The molar ratioof MCC to NC materials is around 0.5. As a typical example for 4 nmInAs—In₂Se₄ ²⁻ NC system, 0.1 ml 0.25 M (N₂H₄)₂(N₂H₅)₂In₂Se₄ solutionwas mixed with 2 ml N₂H₄. 0.16 ml of 0.30 M InAs NC solution in toluenewas diluted with 2 ml hexane and then loaded onto the top of thehydrazine phase. The solution was stirred for around 30 min to 1 houruntil the upper organic phase turned colorless and the lower hydrazinephase turned dark. The organic solution was removed carefully. And thelower hydrazine phase was purified to remove any organics by washingwith anhydrous toluene for three times. Furthermore, the hydrazinesolution was precipitated by minimal amount of anhydrous acetonitrile(approximately 50% of NC solution volume), collected by centrifugation,redissolved into hydrazine and filtered through a 0.2 μm PTFE.

FET Device fabrication. Samples for electrical measurements wereprepared by depositing thin 10 nm-30 nm (±20%) films by spin-coatingMCC-capped NCs onto highly doped Si wafers with a 100 nm thick SiO₂thermal gate oxide. The oxide surface was hydrophilized by piranhasolution prior to NC deposition. The film thickness can be controlled byadjusting concentration of the NC solution and the spinning rate. Allsample preparations were carried out under dry nitrogen atmosphere.As-deposited NC films were dried at 80° C. for 30 min, followed by theannealing at 200° C. for 1 hr. Source and drain Al (˜1000 Å) electrodeswere directly patterned on the annealed NC films using a shadow mask.

Electrical measurements. The electrical measurements were performedusing Agilent B1500A semiconductor parameter analyzer under dry nitrogenatmosphere. All FET measurements were carried out under quasi-staticconditions with typical voltage scan rates of 0.12 Vs⁻¹ and 40 mVs⁻¹ forID-VG and ID-VDS scans, respectively.

The electron mobility was calculated from the slope of the drain current(ID) versus VG measured when the gate voltage was scanned in the forwarddirection (i.e., from negative to positive). For an n-type FET, thismeasurement typically provides a conservative estimate for the fieldeffect mobility.

Photoconductivity measurements. For the photoconductivity measurements,close-packed films of different MCCs (In₂Se₄ ²⁻, Cu₇S₄ ²⁻, and Sn₂S₆ ⁴⁻)capped InAs and In₂Se₄ ²⁻ capped InP NCs were deposited on a Si waferswith a thick layer of 100 nm SiO₂ gate oxide with Ti/Au (5/45 nm)electrodes patterned by photolithography. The surface of the substrateswas hydrophilized by oxygen plasma treatment. As-deposited NC films weredried at 80° C. for 30 min, following by the annealing at 200° C. for 1hr. For the photocurrent vs time (I-t) measurements, the devices werebiased using Keithley 2400 source meter controlled by a LabView program.The photocurrent was measured under illumination with four different CWLED's emitting at 660 nm (1.88 eV), 980 nm (1.27 eV), 1310 nm (0.95 eV),and 1550 nm (0.80 eV), respectively.

For synthesis of hydrazinium chalcogenidometallates (MCCs used here) theMitzi's approach was used based on the dissolution metals or metalchalcogenides in neat N₂H₄ in the presence of elemental chalcogens. Theligand exchange was carried out through a phase transfer procedure inwhich TOP-capped InAs or myristate-capped InP NCs in a nonpolar solventsuch as hexane were transferred into the N₂H₄ phase as the organicligands were replaced with MCCs forming a stable colloidal solution(FIG. 32A). To explore the versatility of the MCC ligand exchange withInAs and InP NCs, five inorganic N₂H₄-MCCs were studied:(N₂H₄)₂(N₂H₅)₂In₂Se₄, (N₂H₄)₃(N₂H₅)₄Sn₂S₆, (N₂H₄)(N₂H₅)₄Sn₂Se₆,(N₂H₄)(N₂H₅)Cu₇S₄, and (N₂H₄)_(x)(N₂H₅)₃{In₂Cu₂Se₄S₃}. TOP-capped InAsNCs worked well with all above MCCs whereas InP NCs capped with myristicacid could be stabilized with all MCC ligands except (N₂H₄)(N₂H₅)Cu₇S₄that apparently etched InP NCs. The purity of the originalorganics-capped NCs was important to success of the ligand exchange andto the performance of the NC-based FET devices. The type of originalorganic ligand on the NCs also influenced the results of the ligandexchange. For instance, InP NCs initially capped with TOP/TOPOprecipitated out in N₂H₄ after ligand exchange with MCCs. Among theseprecipitates, only (N₂H₅)₄Sn₂S₆ capped InP NCs can be redissolved intoformamide (FA) which has much higher dielectric constant (ε=106)compared to N₂H₄(ε=55). The attempts to treat TOP/TOPO capped InP NCswith NOBF₄ to get ligand-free NCs following the approach of Murray etal. and then to perform ligand exchange with MCCs was not successfulbecause NOBF₄ etched the InP NCs. On the other hand, successfulinorganic MCC ligand exchange happened with myristic acid-capped InP NCsleading to the formation of colloidal NCs in N₂H₄. Simple chalcogenideligands (S²⁻, Se²⁻ and Te²⁻) worked well with InAs and InP NCsregardless of original organic surfactants.

The time required to complete the ligand exchange ranged from a fewminutes to several hours. As confirmed by TEM images, the size and shapeof the InAs and InP NCs were preserved after the ligand exchange asshown in FIGS. 32C and 32E. In₂Se₄ ²⁻-capped InAs and Sn₂S₆ ⁴⁻-cappedInP NCs retained their excitonic features in the absorption spectra asshown in FIGS. 32B and 32D, inferring the preservation of NCs sizedistribution. A slight redshift in the first excitonic peak of In₂Se₄²⁻-capped InAs NCs compared to TOP-capped InAs NCs can be explained bypartial relaxation of the quantum confinement due to electron and/orhole wave function leakage to the MCC layer. FTIR spectra taken beforeand after ligand exchange showed the extent of ligand exchange (FIGS.33A, 33B). The strong absorption bands at 2800-3000 cm⁻¹ arising fromcharacteristic C—H stretching were completely diminished after theligand exchange reaction, confirming the complete removal of the organicsurfactants from the surface of the NCs. Two weak absorption linescharacteristic of the N—H stretching appear at 3200-3300 cm⁻¹,indicating new N₂H₄-MCCs attachment to the NCs. The negatively chargedInAs NC surface resulted in a negative zeta potential (FIG. 32F) whichis sufficient to electrostatically stabilize the colloidal dispersion inN₂H₄. A zeta potential of ±30 mV is generally regarded as the borderlinebetween electrostatically stable and unstable colloidal solution inaqueous or similar media.

Dynamic lighting scattering (DLS) measurements shown in FIG. 32F,confirm that the narrow size distribution after ligand exchange issimilar to that of organic capped NCs. A small decrease in the particlesize of the MCC capped NCs compared to the organic ones can beattributed to a smaller hydrodynamic diameter of the MCC capped NCsafter removal of the longer organic ligands. Since all the electric andoptoelectric measurements of inorganic capped InAs and InP NCs wereperformed after 200° C. annealing, various techniques were applied toprobe their morphological and electronic properties before and after theheat treatment. TGA data (FIG. 33E) of (N₂H₄)₂(N₂H₅)₂In₂Se₄ capped InAsNCs showed much less weight loss than the pure ligand at 200° C. Theweight loss at 200° C., was caused by the elimination of weakly boundand thermally unstable N₂H₄ and N₂H₅ ⁺ as confirmed by FTIR data (FIG.33A) that the N—H stretching bands disappear after annealing at 200° C.annealing for 30 min. The X-ray diffraction patterns of(N₂H₄)₂(N₂H₅)₂In₂Se₄ capped InAs and InP NCs were nearly identicalbefore and after 200° C. annealing and showed no reflection of In₂Se₃phase (FIGS. 33C and 33D). The first excitonic absorption peak (FIG.33F) of (N₂H₄)₂(N₂H₅)₂In₂Se₄ capped InAs NCs thin film broadened andslightly blue shifted after annealing at 200° C. for 30 min, indicatingenhanced electronic coupling between nanocrystals and etching of theInAs NCs by the In₂Se₃ ligand.

Charge Transport and Photoresponse in Arrays of MCC-Capped InAs NCs.

The exchange of bulky organic surfactants with small inorganic ligandssuch as In₂Se₄ ²⁻, Cu₇S₄ ²⁻, Sn₂S₆ ⁴⁻ and Sn₂Se₆ ⁴⁻ should facilitatestrong electronic coupling between individual NCs (FIG. 33F). Strongelectronic coupling provides an exciting path for the integration ofcolloidal NCs into electronic and optoelectronic application. For chargetransport studies, source and drain electrodes were deposited on thefilms of MCCs-capped InAs NCs after annealing at 200° C., whichpreserving excitonic features in the absorption spectra of annealedfilms.

FIG. 34 shows the effect of inorganic MCCs ligands on the performance ofFET in closed-packed films of MCC capped of InAs NCs. FIGS. 34A-34Cshows representative drain current (ID) versus drain-source voltage(VDS) scans at different gate voltages (VG) for assemblies a films ofInAs NCs capped with Cu₇S₄ ²⁻, Sn₂S₆ ⁴⁻, and Sn₂Se₆ ⁴⁻ MCCs,respectively. ID increased with increasing VG as characteristic ofn-type transport. The electron mobility corresponding to the linearregime of FET operation measured at VDS=2 V for a film of InAs NCscapped with Cu₇S₄ ²⁻ annealed at 200° C. was μ_(lin)=1.64 cm² V⁻¹s⁻¹with I_(ON)/I_(OFF) 8×10¹ (FIGS. 34A, 34D). The saturation mobility(μ_(sat)) measured at VDS=30V was slightly lower than linear mobility(μ_(lin)), μ_(sat)˜1.3 cm2/Vs. This result expected the high-qualitychannel layers fabricated from Cu₇S₄ ⁻²-capped InAs NC. NH₄Cu₇S₄required a relatively low decomposition temperature (˜120° C.) due tothe relatively weak hydrogen-bonding between the Cu₇S₄ ⁻ anion andthydrozinium/hydrazine species, which allows the formation ofhigh-quality channel layers in FET devices at low annealing process.

On the other hand, FETs assembled from InAs NCs capped with Sn₂S₆ ⁴⁻ andSn₂Se₆ ⁴⁻ ligands annealed at 200° C. showed lower carrier mobility;μ_(lin)=0.2 cm² V⁻¹s⁻¹ with I_(ON)/I_(OFF) 3×10¹ (FIGS. 34B, 34E) andμ_(lin)=0.05 cm² V⁻¹ s⁻¹ with I_(ON)/I_(OFF) 2×10² at VDS=2 V (FIGS.34C, 34F), respectively. The electron mobility measured in thesaturation mode of FET operation (VDS=30V) was much lower than μ_(lin)for InAs NCs capped with Sn₂S₆ ⁴⁻, and Sn₂Se₆ ⁴⁻; 3.6×10⁻⁵ cm² V⁻¹s⁻¹and 4.4×10⁻⁶ cm² V⁻¹s⁻¹, respectively. The measured electron mobilitydepends both on the intrinsic property of the material (particle size,monodispersity etc.) and on a number of parameters related to devicecharacteristics (FET structure, gate dielectric, film uniformity,contacts, etc.). These are all reasons to expect that higher performancewill be obtained after further optimization of the FET devices.

Photoconductivity:

The photoresponse of close-packed films of InAs NCs capped with In₂Se₄²⁻, Cu₇S₄ ²⁻ and Sn₂S₆ ⁴⁻ at room temperatures were measured by fourdifferent excitation sources with respective energies at 660 nm (1.88eV), 980 nm (1.27 eV), 1310 nm (0.95 eV), and 1550 nm (0.80 eV) undernitrogen atmosphere. The dependence of gate voltage on the photocurrentwas also studied for films of InAs NC capped with Sn₂S₆ ⁴⁻ For thesestudies, close-packed films of MCCs-capped InAs NCs were deposited on Siwafers with a thick layer of 100 nm SiO₂ gate oxide and Ti/Au (5/45 nm)electrodes patterned by photolithography, followed by annealing at 200°C. for 1 hr. The gate voltage was applied using a back gate electrode inbottom gate FET configuration (channel length=4.5 μm, W=7800 μm). Thephotoresponse for the colloidal inorganic capped InAs NCs reveals astrong dependence on MCC-ligands. The results show that the coupling ofillumination and gate effect leads to changing the sign of thephotocurrent.

FIG. 35A shows the photocurrent response as a function of excitationenergy in films of In₂Se₄ ²⁻ capped InAs NCs measured at a bias of 2V.The photocurrent shows a positive photoresponse for all excitationenergy. With an increase in excitation energies, more free carriersphotogenerated from the valance band to conduction band can be generatedhigher photocurrent. When the light was off the photocurrent generatedwith a higher energy (>1.27 eV) decayed exponentially with a timeconstant greater than several hundred seconds. FIGS. 35B and 35C showthat the sign of photoreponse of Cu₇S₄ ²⁻ and Sn₂S₆ ⁴⁻-capped InAs NCswas converted from negative at the higher energy excitations of 1.88 eVand 1.27 eV to positive at the lower energy excitations of 0.95 eV and0.80 eV. Negative photocurrent has been observed in several materialsystems such as nanowires, heterostructures, and quantum wellstructures. Without wishing to be bound by theory, the negativephotocurrent was mainly attributed to a reduction in the number ofcarriers available for transport in conduction channel. The responsetime of the negative photogeneration was faster than that of thepositive photoresponse. For an explanation of this phenomena, it ispostulated that the closed-packed films of Cu₇S₄ ²⁻- and Sn₂S₆ ⁴⁻-cappedInAs NCs annealed at 200° C. can be formed high-qualityquasi-multi-quantum well (MQW) structures due to their relatively lowdecomposition temperature. These quasi-MQW structures can provide strongelectron confinement resulting from the large energy difference betweenthe conduction bands and a large type-II valance band offset betweenInAs and Cu₂S (InAs/Cu₂S/InAs) or InAs and SnS₂ (InAs/SnS₂/InAs), whichprovides the formation of a two-dimensional electron gas (2DEG) in theInAs layer. These carriers might produce donor-like defects in thebarriers or donors at the InAs/MCC ligands interfaces. Positivephotoconductivity, which increases the 2DEG density in the conductionchannel layer, is detected upon illumination by photons with excitationenergies lower than about 0.95 eV and might be attributed to thephotoionization of deep donors. On the other hand, negativephotoconductivity occurs upon illumination by excitation energies largerthan about 1.27 eV. The negative photoresponse can be attributed tophotogenerated electron-hole pairs in the large gap layer (MCC layer)followed by spatial charge separation by the built-in field in theheterojunction layer (NC/MCC). The photoexcited electrons are capturedby ionized deep donors in the MCCs barriers, while the holes reachingthe interface of NC/MCC recombine with electrons in the InAs NCs,resulting in a decrease of carrier density in the channel layer.

FIG. 35d represents the gate-dependent photoresponse in films of Sn₂S₄²⁻ capped InAs NCs measured by a function of back gate bias (−20<Vg<20V)at excitation energy of 1.27 eV (980 nm). The effect of applied backgate bias was found to be similar to that observed prior toillumination. The sign of the photocurrent was converted from negativeto positive response by applying a gate bias from −20 Vg to 20 Vg. Byincreasing the negative gate bias (<−20 VG), negative photocurrent wasdecreased and by further increasing the negative bias to −30 Vg, thephotocurrent was completely degenerated due to complete depletion ofcarriers in the inversion layer on the NC/dielectric layer. On the otherhand, when a positive gate bias was applied, the sign of photocurrentchanged above 3 Vg and the positive photocurrent increased withincreasing of positive gate voltage due to the increase of the 2DEGsheet density.

FIGS. 36A-36B shows that FETs assembled from colloidal InP NCs cappedwith In₂Se₄ ²⁻ show poor charge transport behavior even after annealingat 250° C. for 30 min. FIGS. 36C-36D show photoresponse for a film ofIn₂Se₄ ²⁻ capped InP NCs. Upon illumination an excitation source abovethe band gap of InP (E_(g)=1.27 eV at 300K), the photocurrent sharplyincreased with respective energies at 1.88 eV and 1.27 eV. On the otherhand, photocurrent in films of In₂Se₄ ²⁻ capped InP NCs was notgenerated when lower energy excitation sources.

Examples

Chemicals:

Potassium sulfide (anhydrous, ≥95%, Strem), sodium sulfide nonahydrate(98%, Aldrich), ammonium sulfide (40-48 wt % solution in water,Aldrich), Potassium hydroxide (≥90%, Aldrich), Sodium amide (Aldrich),Sulfur (99.998%, Aldrich), Selenium (powder, 99.99%, Aldrich), tellurium(shot, 99.999%, Aldrich), formamide (FA, spectroscopy grade, Aldrich),dimethylsulfoxide (DMSO, anhydrous, 99.9%, Aldrich), acetonitrile(anhydrous, 99.8%, Aldrich), didodecyldimethylammonium bromide (DDAB,98%, Fluka), Trioctylphosphine oxide (TOPO, 99%, Aldrich),tetradecylphosphonic acid (TDPA, 99%, Polycarbon), octadecylphosphonicacid (ODPA, 99%, Polycarbon), dimethylcadmium (97%, Strem), Cadmiumoxide (99.995%, Aldrich), diethylzinc (Aldrich), trioctylphosphine (TOP,97%, Strem), dodecanethiol (98%, Aldrich), oleylamine (Aldrich),tert-butylamine-borane (97%, Aldrich), InCl₃ (99.99%, Aldrich), Indiumacetate (99.99%, Alfa Aesar), Al₂Se₃ (95%, Strem) and Al₂Te₃ (99.5%,CERAC Inc.).

Nanocrystal Synthesis.

Zinc blende phase CdSe NCs capped with oleic acid and oleylamine wereprepared following Cao et al. Wurtzite phase CdSe NCs capped withn-octadecylphosphonic acid (ODPA) were prepared from CdO and TOPSe.Large, 12 nm CdSe NCs capped with n-tetradecylphosphonic acid (TDPA)were synthesized using Cd(CH₃)₂ and TOPSe as precursors. CdSe andCdSe/ZnS core/shell NCs capped with proprietary ligands obtained fromEvident Technologies Inc. (Troy, N.Y.) were also used.

To synthesize InP NCs, InCl₃ solution (1.036 g InCl₃ in 3.44 mL TOP),075 g P(SiMe₃)₃, 7.4 g TOP and 0.9 g TOPO were mixed at roomtemperature. The mixture was heated at 270° C. for 2 days, and cooleddown to room temperature. Post-synthesis size selective precipitationwas carried out using toluene and ethanol as solvent and non-solvent,respectively.

InAs NCs with TOP as capping ligands were prepared from InCl₃ andAs[Si(CH₃)₃]₃.

Au NCs were synthesized following Stucky et al. 0.25 mmol AuPPh₃Cl wasmixed with 0.125 mL of dodecanethiol in 20 mL benzene forming a clearsolution. 2.5 mmol of tert-butylamine-borane complex was then added tothe above mixture followed by stirring at 80° C.

ZnSe NCs were prepared with some modifications. 7 mL oleylamine wasdegassed at 125° C. under vacuum for 30 min and then heated to 325° C.under N₂ flow. The solution containing 0.5M Zn and Se precursors wasprepared by co-dissolution of diethylzinc and Se in TOP at roomtemperature. 2 mL of the precursors solution was added to degassedoleylamine at 325° C. One mL of additional precursors solution was addedto the reaction mixture after 1 h, followed by two successive injectionsof 1.5 mL precursors solution after 2.5 and 4 hrs. The reaction wascontinued for 1 h after the final injection of the precursors and thencooled to room temperature. The NCs were washed by precipitation withethanol and redispersed in hexane. The washing procedure was repeatedthree times.

CdS nanorods were prepared by slightly modified procedure. 0.207 g CdO,1.08 g n-ODPA, 0.015 g n-propylphosphonic acid, and 3.35 g TOPO wereheated at 120° C. under vacuum for 1 h, followed by heating the mixtureto 280° C. under N₂ until the formation of a clear solution. The mixturewas degassed at 120° C. for 2 h before it was heated to 300° C. underN₂. 2 g TOP was injected into the mixture at 300° C. and temperature wasimmediately set to 320° C. 1.30 g TOPS (TOP:S=1:1) was injected at 320°C. and heating was continued for 2 h. The reaction mixture was cooled toroom temperature, washed twice with toluene/acetone and redispersed intoluene.

In₂O₃ NCs were prepared using the recipe of Seo et al. by heating aslurry of In(OAc)₃ and oleylamine at 250° C.

CdTe NCs were synthesized similar to the recipe published.

PbS NCs capped with oleic acid were synthesized according to theprotocol developed by Hines et al.

Synthesis of Inorganic Ligands.

Sulfides, hydrogen sulfide, hydroxides and amide ligands were purchasedfrom Aldrich and Strem and used as received.

K₂Se was synthesized by the reaction of K (25.6 mmol) with Se (12.8mmol) in about 50 mL liquid ammonia. Special care was taken to make surethat the chemicals and reaction environment were air and moisture free.A mixture of dry ice and acetone was used to liquefy NH₃ gas. Gray-whiteK₂Se powder was stored inside a glovebox. In air K₂Se turned red becauseof the formation of polysenides. K₂Te was prepared in the same way asK₂Se. 7.6 mmol of K and 3.8 mmol of Te were used.

0.05 M solutions of KHSe and KHTe in formamide (FA) were prepared bytitrating 25 mL of 0.05 M KOH solution with H₂Se (or H₂Te) gas generatedby the reaction of Al₂Se₃ (or Al₂Te₃) with 10% H₂SO₄. 1.7 fold molarexcess of H₂Se (or H₂Te) was used. A rigorous N₂ environment wasmaintained while handling the KHSe and KHTe solutions.

(NH₄)₂TeS₃ was synthesized using a modified literature method of Gerl etal. 2TeO₂*HNO₃ was prepared by dissolving 5 g Te (pellets, Aldrich) in35 mL HNO₃ (65%, aqueous) diluted with 50 mL H₂O. The solution wasboiled in an open beaker until volume decreased to about 30 mL. Uponcooling, 2TeO₂*HNO₃ was precipitated as a white solid that was separatedby filtering, rinsed with deionized water and dried. To prepare(NH₄)₂TeS₃, 2 g of 2TeO₂*HNO₃ was mixed with 40 mL aqueous ammoniasolution (30% NH₃, Aldrich). Solution was first purged with N₂ for 5min, then with H₂S until all telluronitrate dissolved forming a yellowsolution, characteristic for TeS₃ ²⁻ ions. Solvents and (NH₄)₂S wereremoved by vacuum evaporation. The solid was redispersed in 36 mL H₂O, 4mL NH₄OH and 0.1 mL N₂H₄ (used as a stabilizer against oxidativedecomposition) forming a clear yellow solution of TeS₃ ²⁻ ions (withsmall amounts of Te₂S₅ ²⁻ as confirmed by ESI-MS) at a concentration ofabout 0.25M.

Ligand Exchange.

Ligand Exchange with Chalcogenide (S²⁻, Se²⁻, Te²⁻) andHydrochalcogenide (HS⁻, HSe⁻, HTe⁻) Ions.

The ligand exchange process was typically carried out under inertatmosphere. Colloidal dispersions of different NCs with organic ligandswere prepared in nonpolar solvents like toluene or hexane whilesolutions of inorganic ligands were prepared in polar formamide (FA)immiscible with toluene and hexane. For a typical ligand exchange usingS²⁻ ions, 1 mL CdSe NC solution (2 mg/mL) was mixed with 1 mL of K₂Ssolution (5 mg/mL). The mixture was stirred for about 10 min leading tocomplete phase transfer of CdSe NCs from toluene to FA phase. The phasetransfer can be easily monitored by the color change for toluene (red tocolorless) and FA (colorless to red) phases. The FA phase was separatedout followed by triple washing with toluene to remove any remainingnon-polar organic species. The washed FA phase was then filtered througha 0.2 m PTFE filter and 1 mL acetonitrile was added to precipitate outthe NCs. The precipitate was re-dispersed in FA and used for furtherstudies. The NC dispersion in FA was stable for months. Ligand exchangewith Se²⁻, Te²⁻, HS⁻, HSe⁻ and HTe⁻ ligands were carried out in asimilar manner. In some cases, the ligand exchange can take longer time,up to several hours.

The exchange of organic ligands with S²⁻ and SH⁻ can be carried out inair as well. Moreover, one can use concentrated aqueous solutions of(NH₄)₂S, K₂S and Na₂S as S²⁻ source to carry out the ligand exchange. Asan example, 10 μL of (NH₄)₂S solution (Aldrich, 40-48 wt % in water) wasadded to 1 mL FA and mixed with NC dispersion in toluene or hexane. Therest of the ligand exchange procedure was similar to the above protocol.When handled in air, the solutions of S²⁻ capped NCs preserve theircolloidal stability for only several days. Resulting precipitates can beeasily re-dispersed after addition of ˜5 μL (NH₄)₂S solution; thedispersions stabilized with additional S²⁻ remained stable for severalweeks in air. Similar to water and FA, the ligand exchange can becarried out in DMSO.

Ligand Exchange with TeS₃.

The mixed chalcogenide Te—S species are stable in basic solutions,(NH₄OH was typically used) and are highly susceptible to oxidation. Allligand-exchange reactions were carried out in a glovebox. In a typicalligand exchange for CdTe NCs, 0.4 mL CdTe NCs capped with oleic acid intoluene (˜25 mg/mL) was mixed with 3 mL FA, 3 mL toluene, and 0.4 mL0.25M (NH₄)₂TeS₃ solution. Upon stirring for 2-10 hours, CdTe NCsquantitatively transferred into FA phase. NC solution was rinsed 3 timeswith toluene and mixed with 3-6 mL acetonitrile. NCs were isolated bycentrifuging and redispersed in FA.

Ligand Exchange with OH⁻.

A stock solution of KOH was prepared by dissolving 135 mg KOH in 0.4 mLFA. For a typical ligand exchange reaction, ˜2 mg 5.5 nm CdSe NCs weredispersed in 1 mL toluene. 1 mL FA was added to the NC solution followedby the addition of 20 μL KOH solution and stirred for about 10 min. Redcolored FA phase was separated out, washed three times with toluene andpassed through a 0.2 m PTFE filter. Acetonitrile was added toprecipitate the NCs, followed by centrifugation and re-dispersion of NCsin FA. The process was carried out under inert atmosphere.

Ligand Exchange with NH₂.

0.05 g NaNH₂ was dissolved in 0.5 mL FA to prepare a stock solution.Then, 0.1 mL of the NaNH₂ stock solution was diluted to 1 mL by addingFA and the resulting solution was added to 1.5 mg CdSe NCs dispersed in1 mL toluene. The mixture was stirred for about 10 min. The NCs wereseparated out to the FA phase, washed, precipitated, centrifuged andre-dispersed in FA under inert atmosphere.

Treatment with HBF₄ and HPF₆.

1.5 mg CdSe NCs was dispersed in 1 mL toluene followed by the additionof a solution of HBF₄ (prepared by mixing 25 μL of 50 wt % aqueous HBF₄with 1 mL FA). NCs completely transferred to the FA phase within 5minutes. The colorless toluene phase was discarded followed by theaddition of pure toluene to wash out any remaining non-polar organics.The washing was repeated 3-4 times in about 30 minutes and finally theNCs dispersed in FA were passed through a 0.2 μm PTFE filter. HPF₆treatment was carried out in similar manner. All these steps werecarried out under the nitrogen environment inside a glove box, and theFA dispersion of CdSe NCs obtained after the ligand exchange was foundto be stable for a few days, after which the NCs precipitates out fromthe solution.

Characterization.

UV-vis absorption spectra of NC dispersions were recorded using a Cary5000 UV-vis-NIR spectrophotometer. Photoluminescence (PL) spectra werecollected using a FluoroMax-4 spectrofluoremeter (HORIBA Jobin Yvon). PLquantum efficiency (QE) of the NC dispersions were measured usingRhodamine 6G (QE=95%) as a reference dye dissolved in ethanol.Fourier-transform infrared (FTIR) spectra were acquired in thetransmission mode using a Nicolet Nexus-670 FTIR spectrometer. Samplesfor FTIR measurements were prepared by drop casting concentrated NCdispersions on KBr crystal substrates (International CrystalLaboratories) followed by drying at 90° C. under vacuum. Transmittanceof different NC samples was normalized by the weight of NCs per unitarea of the deposited film, assuming a uniform film thickness across theKBr substrate. Transmission Electron Microscopy (TEM) data were obtainedusing a FEI Tecnai F30 microscope operated at 300 kV. Dynamic lightscattering (DLS) and ζ-potential data were obtained using a ZetasizerNano-ZS (Malvern Instruments, U.K.). ζ-potential was calculated from theelectrophoretic mobility using Henry's equation in the Smoluchowskilimit. Electrospray ionization mass spectrometry (ESI-MS) was performedusing Agilent 1100 LC/MSD mass-spectrometer. The peak assignments werebased on the comparison of experimental mass-spectra with calculatedisotope patterns.

Charge Transport Studies. Device Fabrication.

All fabrication steps were carried out under dry nitrogen atmosphere.Highly p-doped Si wafers (Silicon Quest, Inc) with a 100 nm thick layerof thermally grown SiO₂ gate oxide were used as a substrate. The oxidelayer on the back side of the wafers was etched using 1% HF aqueoussolution, followed by careful rinsing with high purity deionized water.The substrates were cleaned with acetone, iso-propanol and ethanol,followed by drying in a flow of N₂ gas. SiO₂ surface was hydrophilizedby oxygen plasma or piranha (H₂SO₄:H₂O₂) treatments immediately prior toNC deposition. The substrates were treated by oxygen plasma (˜80 W) for10-15 min. Piranha treatment was usually carried out at 120° C. for 10minutes followed by multiple washings with high purity deionized water.Piranha solution was prepared by adding concentrated sulfuric acid to30% H₂O₂ solution (2:3 volume ratio).

The NC films for electrical measurements were prepared by depositingthin 20-40 nm films by spin-coating (spread: 600 rpm, 6 s; spin: 2000rpm, 30 s) from FA solutions at elevated temperatures (80° C.) using aninfrared lamp placed above the substrate. After deposition, the NC filmswere dried at 80° C. for 30 min, followed by the annealing at 200-250°C. for 30 min using a hot plate with calibrated temperature controller.The NC film thickness was measured using AFM or high resolution SEM ofdevice cross-sections. The top source and drain electrodes werethermally evaporated at 1.0-2.5 Å/s rate up to ˜1000 Å total thicknessusing a metal evaporator located inside nitrogen-filled glove box. Afterdeposition of Al electrodes, the devices were post annealed at 80° C.for 30 min to improve the contacts.

Electrical Measurements.

All electrical measurements were performed using Agilent B1500Asemiconductor parameter analyzer. The source electrode was grounded forthe field-effect transistor measurements. All electrical measurementswere performed under dry nitrogen atmosphere and under quasi-staticconditions with slow scan of source-drain or source-gate voltage.

CdSe(zb)/CdS and CdSe(w)/CdS:

FIG. 27 shows the comparison between growth made on zinc blende CdSe NCsand wurtzite CdSe NCs. The XRay pattern of the cores confirm the twostructures. Absorption spectra of CdSe-xSCd with x=1 . . . 10 and TEMimages of CdSe-10SCd for two different ways to grow the shell: (Amiddle) with (NH₄)₂S as a precursor of sulfide and cadmium oleate,Cd(OA)₂, as a precursor of cadmium, and the nanocrystals are washedafter each monolayer; (B bottom) QDs are stabilized by oleylamine eitherwhen they are sulfide rich or cadmium rich. Cd(OAc)₂ and (NH₄)₂S areused as precursors of cadmium and sufide. TEM images of each sample,scale bar=20 nm.

CdS/ZnS Doped with Manganese:

FIG. 31 shows absorption spectra (color) of CdS-xSZn doped withmanganese in the third layer for x=1 . . . 6 and emission spectrum ofCdS-6SZn:Mn showing the phosphorescence coming from Mn²⁺.

As discussed above colloidal particles may be cross-linked (i.e.,bridged) using metal ions, whereby one or more metal ions individuallybinds, ionically or covalently, binding sites on the exterior of aplurality of colloidal particles. The binding sites may be parts of theinorganic capping agent disposed perpendicular to a surface on thenanoparticle. Various example bridging configurations between colloidalparticles are described herein using cationic ions, preferably cationicmetal ions. And, as described, selection of the bridging cationic ionsmay be chosen to adjust various properties of the colloid particles. Asdescribed the cationic ion may be a transition metal ion, a main groupion, a lanthanide ion, or an actinide ion. Preferably, the cationic ionis selected from those ions that can impart advanced electronic,magnetic (e.g., Mn²⁺, Co²⁺), or photophysical properties on thecross-linked colloidal particles. Still more preferably the cationic ionis Pt²⁺, Zn²⁺, Cd²⁺, Mn²⁺, Mn³⁺, Eu²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ge⁴⁺, Cu²⁺,Cu⁺, Pb²⁺, Bi³⁺, Sb³⁺, In³⁺, Ga³⁺, Au⁺, Au³⁺, Ag⁺, Sn²⁺, Sn⁴⁺, or Hg²⁺.The ability to adjust such properties allows for tailored selectivity ofcolloidal particles, in particular colloidal nanocrystals (NCs) whichprovide convenient “building blocks” for various electrical,electromechanical, magnetic, electrooptic structures, photovoltaics, andthermoelectrics, including solar cells, light-emitting devices,photocatalytic systems, etc. The use of inorganic ligands for colloidalNCs dramatically improves inter-NC charge transport, enabling fastprogress in NC-based devices.

As described herein, the cationic part of inorganic ligands may be usedto systematically engineer the NC properties and to impart additionalfunctionalities to all-inorganic colloidal NCs. And in this way, cationsprovide a convenient tool for tailoring optical, electronic, magneticand catalytic properties of all-inorganic NCs and NC solids. In examplesdiscussed herein, cationic metal ions such as Cd²⁺, Mn²⁺, In³⁺ etc. canbind to the NC surface in solution and inverse the surface charge of thecolloidal particles from negative to positive, along with dramaticimprovement of luminescence efficiency. In a NC film, for example,cations can link adjacent NCs and improve electron mobility. Cationicmetal ions (e.g., K⁺, In³⁺) can switch conductivity in CdTe NC solidsfrom p-type to ambipolar to n-type. Cationic metal ions Mn²⁺ can bridgeS²⁻ capped CdSe NCs via S—Mn—S bonds and impart magnetic properties tooriginally diamagnetic CdSe NCs. Bridging semiconductor NCs with Pt²⁺ions offer control over the catalytic properties of NC solids. Exampleempirical NC synthesis and cationic ion bridging and correspondingeffects and effect on colloidal NC properties are now described.

Nanocrystal Synthesis.

4.4 nm Zinc blende CdSe NCs capped with oleic acid were preparedfollowing the techniques as described by Cao et al. 4.2 nm Wurtzite CdSeNCs capped with octadecylphosphonic acid (ODPA) were prepared from CdOand n-trioctylphosphine selenide (TOPSe) following Manna, et al. Asdescribed below the Zinc blende NCs and the wurtzite NCs showed similarresults in various studies. In addition, CdSe/ZnS core/shell NCs cappedwith proprietary amine-based ligands obtained from Evident TechnologiesInc. (Troy, N.Y.) were used. InAs NCs capped with trioctylphosphine(TOP) were prepared by reacting InCl₃ and As[Si(CH₃)₃]₃. CdS nanorodswere prepared. CdTe NCs capped with tetradecylphosphonic acid (TDPA)were then prepared.

Ligand Exchange.

The exchange of original organic ligands with S²⁻ and Te²⁻ was carriedout per the foregoing. For example, in a typical experiment, 1 mL (5mg/mL) organics-capped CdSe NCs dispersion in toluene was combined with1 mL K₂S solution (12 mg/mL) in formamide (FA), stirred for about 10 minleading to a complete phase transfer of NCs from toluene to the FAphase. The FA phase was separated out and washed three times withtoluene. NCs were then precipitated by adding a minimum amount (0.8 mL)of acetonitrile, centrifuged and re-dispersed in FA. A similar ligandexchange with Na₂Sn₂S₆ was carried. All ligand exchange reactions werecarried out in a glovebox filled with dry nitrogen.

NC Charge Inversion and Photoluminescence (PL) in Solution.

For studies of charge inversion and PL efficiency, 1 mL dilute solution(0.3 mg/mL) of K₂S capped CdSe or CdSe/ZnS NCs in FA was combined with 2mL FA solution of a salt containing desired cation and an anion with lownucleophilicity (NaNO₃, KNO₃, Cd(NO₃)₂, Zn(NO₃)₂, Ca(NO₃)₂, Mg(NO₃)₂,Cd(ClO₄)₂, Ba(NO₃)₂, In(NO₃)₃, or Al(NO₃)₃, etc). To ensure colloidalstability, all NC solutions were filtered through 0.2 m PTFE syringefilter prior to optical measurements. The NC solutions retained theircolloidal stability in the presence of some metal ions for only severaldays and freshly prepared mixtures of NCs and metal ions for all opticaland zeta-potential measurements were used. The solutions were typicallyhandled inside a N₂ glovebox.

Linking NCs with Inorganic Cations and Fabrication of FET Devices.

20 nm thick films of inorganically-capped NCs were prepared byspin-coating (2000 rpm, 60 s) solutions of K₂S capped CdSe NCs (˜20mg/mL) in FA on a suitable substrate at an elevated temperature (80° C.)using an infrared lamp placed above the substrate. Quartz substrateswere used for UV-visible measurements and heavily p-doped Si wafers(Silicon Quest, Inc) with a 100 nm thick layer of thermally grown SiO₂gate oxide were used for FET measurements. Substrates were hydrophilizedby oxygen plasma or piranha (H₂SO₄:H₂O₂) treatments. Dried NC films weredipped into 50 mM solutions of metal salts (Cd(NO₃)₂, Zn(NO₃)₂,In(NO₃)₃, and Mn(NO₃)₂) in methanol for 30 min followed by washing thefilms in fresh methanol. For FET measurements, the cation treated NCfilms were annealed at 200° C. for 30 min to remove solvents andremaining volatile ligands without sintering the NCs. The NC filmthickness was measured using AFM. The top source and drain electrodes(Al) were thermally evaporated at 1-2 Å/s rate up to ˜1000 Å totalthickness using metal evaporator installed inside a glovebox. The FETchannel length was 150 m for the FET devices with bottom gate and topsource-drain electrodes. After deposition of top electrodes, the deviceswere annealed at 80° C. for 30 min to improve the contacts. All stepswere carried out under N₂ atmosphere inside a glovebox. For the CdTe NCdevices, both bottom gate/top source-drain FET geometry (channel length150 m) and bottom gate/bottom source-drain FET geometry (channel length5 m) with Au or Al source and drain electrodes, were examined.

Electrical measurements were carried out under quasi-static conditionswith slow scan of source-drain or source-gate voltage using AgilentB1500A semiconductor parameter analyzer and home-made probe stationplaced inside a N₂-filled glovebox. The transfer characteristics for theFETs were measured with the gate voltage (V_(G)) sweep rate 40 mV/s. Thelinear regime mobility (μ_(lin)) and the saturation regime mobility(μ_(sat)) were calculated from FET transfer characteristics as describedbelow.

Characterization.

Continuous wave EPR experiments were carried out using a Bruker ELEXSYSE580 spectrometer operating in the X-band (9.4 GHz) mode and equippedwith an Oxford CF935 helium flow cryostat with an ITC-5025 temperaturecontroller. Magnetic properties were studied using SQUID Magnetometry(MPMS XL, Quantum Design). Zero-field-cooled (ZFC) and field-cooled (FC)studies were recorded in the 5-300K range at H=100 Oe after cooling theNC samples in zero field or in a 100 Oe field, respectively. Mn K-edgeEXAFS measurements were carried out at the MRCAT 10-ID beamline at theAdvanced Photon Source, Argonne National Laboratory (ANL) in thefluorescence mode with ionization chamber in Stern-Heald geometry forthe NC samples. Samples were loaded in a teflon sample holder inside anN₂ glovebox and sealed inside an air-tight chamber. Mn standard sampleswere measured in the transmission mode. The sulfur K-edge XANES spectrawere measured at the 9-BM beamline at the Advanced Photon Source in thefluorescence mode. The EXAFS data were processed using Athena softwareby extracting the EXAFS oscillations χ(k) as a function of photoelectronwave number k following standard procedures. The theoretical paths weregenerated using FEFF6 and the models were fitted in the conventional wayusing Artemis software. Artemis was used to refine the fittingparameters used for modeling each sample in the R-space until asatisfactory model describing the system was obtained. For EPR, magneticand EXAFS/XANES measurements, samples were prepared in form ofthoroughly washed and dried powders of CdSe/S²⁻ nanocrystalscross-linked with Mn²⁺ ions. Electrochemical experiments were carriedout using a Gamry Reference 600 potentiostat using a three-electrodeelectrochemical cell. For electrochemical measurements in anhydrousacetonitrile, 0.1 M tetrabutylammonium perchlorate (TBAP) was used as anelectrolyte. Glassy carbon (GC), platinum wire and Ag⁺/Ag (0.01 M AgNO₃in CH₃CN) electrodes were used as the working, counter and referenceelectrodes, respectively. The NC films were formed at the surface of aglassy carbon electrode by drop casting. For optical studies,ITO-covered glass was used as the working electrode with a NC filmprepared by spin-coating. The electrochemical measurements inacetonitrile were carried out in presence of dimethylformamide (DMF)protonated by trifluoromethanesulfonic (triflic) acid. Protonated DMFwas synthesized. Equimolar amounts of triflic acid and DMF were mixedand stirred for 2-3 minutes resulting in formation of white solidsoluble in acetonitrile and dichloromethane. Typical [H(DMF)]⁺OTf⁻concentration used in this work was 0.01 M.

For electrochemical measurements in aqueous solutions, pH was bufferedat 6.50 by a phosphate buffer (a mixture of K₂HPO₄ and KH₂PO₄ with totalconcentration of phosphate 0.15 mol/L). Glassy carbon (GC), platinumwire and Ag/AgCl (saturated KCl solution) electrodes were used as theworking, counter and reference electrodes, respectively. The area of GCelectrode was 7.07.10⁻² cm². The NC films were prepared at the surfaceof glassy carbon electrode by drop casting. All solutions were carefullydegassed and kept under nitrogen atmosphere.

Charge Inversion of Inorganically-Capped Nanocrystals.

4.2 nm CdSe NCs were transferred from toluene into a polar solvent(formamide, FA) by treatment with K₂S that exchanged originaln-tetradecylphosphonate surface ligands with S²⁻. The completeness ofligands exchange was confirmed by FTIR measurements showing completeremoval of the absorption bands around 3000 cm⁻¹ corresponding to C—Hstretching in the hydrocarbon tails of the organic ligands (FIG. 46).S²⁻ covalently bonded to the NC surface (FIG. 37A) resulting innegatively charged CdSe NCs with measured electrophoretic mobility−0.7×10⁻⁸ m²V⁻¹s⁻¹ and ζ-potential −35 mV calculated using Henry'sequation. By knowing the ionic strength, one could also calculatesurface charge density (o). For a spherical particle immersed in anelectrolyte, σ=ε₀εζ(1+κr)/r, where ε₀ is the vacuum permittivity, ε (109for FA) is the dielectric constant of solvent, r is a hydrodynamicradius of NC and k⁻¹ is Debye screening length of the electrolyte. Thecorresponding average surface charge of a spherical NC, Z, in units ofelementary charges (e) can be obtained as Z=4πr²σ/e. For K₂S capped CdSeNCs (CdSe/S²⁻ NCs, ζ=−35 mV), σ=−0.016 C/m² was obtained or ˜9 negativeelemental charges accumulated at the surface of CdSe NCs.

Negatively charged chalcogenide ions bound to the surface of CdSe NCsshould electrostatically interact with nearby cations. In addition, theymay exhibit specific chemical affinity toward metal ions forming strongchemical bonds to chalcogens (FIG. 37A). In a polar solvent, theseinteractions can be monitored by measuring changes of electrophotericmobility in the presence of specific cations. It was found that additionof 5 mM of various metal salts to a dilute (0.3 mg/mL) solution ofCdSe/S²⁻ NCs results in significant changes of ζ-potential. FIG. 37B,for example shows that ζ-potential of 4.2 nm CdSe NCs in FA can be tunedalmost continuously from −35 mV for CdSe/S²⁻ to +28 mV for CdSe/S²⁻/Cd²⁺depending upon the chemical nature of added cation. FIG. 38B shows datafor ζ-potentials in presence of 5 mM metal ion solution for differentmetal ions. In FIG. 47, the concentrations of each M(NO₃)_(n) salt wereadjusted to the total ionic strength of 15 mM (corresponding to 5 mMconcentration of divalent metal nitrate, n=2), revealing the same trendof ζ-potentials. When K⁺ or Na⁺ were added to the NC solutions in formof nitrates, the absolute value of ζ-potential decreased from −35 mV toabout −20 mV which could be explained as an increase of the ionicstrength without specific binding of metal ions to the NC surface. Thisscenario agrees with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theorythat takes into account Coulombic interactions of colloidal particleswith point-like charges. However, the DLVO theory cannot explain theinteraction of CdSe NCs with other cations, such as Al³⁺, In³⁺, Zn²⁺,Ca²⁺ and Cd²⁺ that result in the charge inversion as shown in FIG. 37B.For example, the addition of 5 mM Cd²⁺ in the form of Cd(NO₃)₂ orCd(ClO₄)₂ to 0.3 mg/mL solution of 4.2 nm CdSe/S²⁻ NCs in FA resulted in+28 mV ζ-potential. This corresponds to ˜11 elemental charges ((=+0.018C/m²) associated with CdSe/S²⁻ NCs exposed to 5 mM Cd²⁺ solution. Afteraddition of Mg²⁺, Ba²⁺, Al³⁺ or In³⁺ to CdSe/S²⁻ NCs, the magnitude ofζ-potential is less than 15 mV, resulting in a limited colloidalstability. Charge inversion was observed in case of CdSe NCs capped withMCC ligands such as Sn₂S₆ ⁴⁻ (FIG. 48).

In general, charge inversion does not require a specific binding ofopposite charges. It can result from the effect of correlation betweenmultiple charged ions near the interface, not accounted by themean-field theories such as DLVO. For example, it has been shown thatscreening of a strongly charged particle by multivalent counterions canresult in the inverted charge Q*with magnitude Q*≈0.83√{square root over(QZe)}, where Q is the original particle charge, Ze is the charge of thecounter ions. This would predict 4.2 nm CdSe NCs with Q=−9e maximuminverted charge of Q*=3.5e after treating with divalent cations. Thepredicted Q* is significantly smaller than the inverted charge (Q*=11e)calculated from the measured electrophoretic mobility of CdSe/S²⁻/Cd²⁺NCs in presence of 5 mM Cd²⁺ ions. In addition, FIG. 37B shows thathighly charged cations (Al³⁺, In³⁺) generate smaller charge inversionthan Ca²⁺, Zn²⁺ and Cd²⁺. Moreover, the chosen cationic metal ion mayvary the amounts of charge inversion experienced. For example, theaddition of Ca²⁺ ions to cross-link colloidal NCs results in a strongcharge inversion, while Mg²⁺ and Ba²⁺ with similar charge and chemicalproperties do not show this effect at all (see, e.g., FIG. 37B, inset).Without wishing to be bound by theory, these observations suggest thatcharge inversion of CdSe/S²⁻ NCs cannot be explained by electrostaticmechanism, but rather requires analysis of chemical interactions betweenNCs and metal ions. Indeed, the addition of Cd²⁺ ions to CdSe NCsresults in a small but reproducible (˜4 nm or ˜16 meV) red shift of theexcitonic peaks in UV-Vis and photoluminescence (PL) spectra (FIG. 37C),consistent with a small increase in the effective NC diameter due tosurface binding of Cd²⁺. Other metal ions resulted in even smallershifts of the absorption maximum, suggesting cation exchange reactionsdo not occur inside the NCs (FIG. 49).

The charge inversion should be facilitated by high affinity of thecounter ion to NC surface and by lower solvation energy of the counterion. Herein, there are indications that softer cations have higheraffinity (Cd²⁺>Zn²⁺; In³⁺>Al³⁺) to CdSe/S²⁻ NC surface exposing softbasic sites like S^(δ−) and Se^(δ−). Generally, cation hardness andsolvation energy in polar solvents decrease down the group in theperiodic table, thus the charge inversion efficiency for CdSe/S²⁻ NC isexpected to follow the trend Mg²⁺<Ca²⁺<Ba²⁺. However, among thesechemically-similar cations, only Ca²⁺ induced charge inversion, whereasaddition of Mg² and Ba²⁺ resulted in aggregation of NCs (FIG. 37B). Toexamine this disparity, ionic radii of metal ions were compared to theradius of Cd²⁺ ions as shown in the inset to FIG. 37B. As shown, Mg²⁺ions are too small and Ba²⁺ is too large to efficiently integrate intosurface cites of CdSe lattice, while the ionic radius of Ca²⁺ is onlyabout 5% larger than that of Cd²⁺, offering an excellent match to theCdSe lattice. This observation suggests that, in addition to chemicalaffinity, steric factors play an important role in the interaction ofmetal ions with surface of all-inorganic NCs. For a colloidal solutionof crystalline particles, similar size of the counter ion (ionic radiusof Mg²⁺<Cd²⁺˜Ca²⁺<Ba²⁺) compared to the parent cation (Cd²⁺ in CdSe/S²⁺)can significantly facilitate charge inversion.

The Correlation of Charge Inversion and PL Efficiency.

Through these examples, a correlation between ζ-potential and PLefficiency was revealed, as shown in FIG. 37D. The metal ions providinglarge charge inversion also resulted in a large increase of PLefficiency. For example, a 25-fold increase of PL efficiency from 0.7%for CdSe/S²⁻ to 17.3% was observed upon addition of 5 mM Cd²⁺ ions. ThePL efficiency is a very sensitive probe of the NC surface. Withoutwishing to be bound by theory, it is believed this correlation betweensurface charge and PL efficiency results, at least in part, from reducedconcentration of surface trap states. The increase of PL efficiency mayalso be explained by binding metal ions to the surface of CdSe/S²⁻ NCs.At the same time, the correlation and effects on PL efficiency arecounterintuitive and do not align with current understanding of surfacepassivation for traditional CdSe NCs. First, it was found that Cd²⁺ andCa²⁺ ions provided higher PL efficiency than Zn²⁺, despite the fact thatZnS is known as an excellent material to passivate the surface of CdSeNCs confining both electron and hole in the core. Furthermore, the sametrend was observed when metal ions were added to CdSe/ZnS/S²⁻NCs. Theaddition of Cd²⁺ and Ca²⁺ ions, for example, reproducibly yielded higherPL efficiency than the addition of Zn²⁺ (FIGS. 38A and 50). This trendwas observed for several batches of CdSe/ZnS core-shells synthesizedin-house or obtained from different commercial sources. The addition ofCd²⁺ or Ca²⁺ increased PL efficiency to nearly 60%, which was close tothe efficiency of original, organically capped NCs. The possibleexplanation is that Zn²⁺ ions are strongly solvated by FA molecules andtherefore show weaker affinity to the surface of CdSe or CdSe/ZnS NCs.

Time-resolved PL decay studies for CdSe/ZnS NCs before and aftertreatment with Cd²⁺ ions suggest that observed PL increase resulted fromthe suppression of a non-radiative recombination channel introduced byS²⁻ ligands (FIG. 37B). In presence of 5 mM Cd²⁺ ions, the PL decay ofCdSe/ZnS NCs closely matched that of the original organics-capped NCs(FIG. 37B).

These results also point to the potential of calcium salts as shellmaterial for luminescent colloidal NCs. It should be noted that Ca²⁺typically forms solids with octahedrally coordinated Ca²⁺ sites, insteadof tetrahedrally coordinated Cd²⁺ sites in CdSe lattice. Therefore, Ca²⁺can be used as a shell material for lead chalcogenide NCs. In additionto chemical passivation of surface dangling bonds, positively chargedions adsorbed at the NC surface can generate an electric field pushingholes away from the NC surface.

Cation Cross-Links (i.e., Bridges) Determine Electron Mobility inall-Inorganic Nanocrystal Arrays.

Thus, the foregoing provides further examples that metal ions, such asCd^(2+,) can bind directly to the surface of CdSe/S²⁻ NCs resulting in acharge inversion in dilute (˜0.1-0.3 mg/mL) colloidal solutions.However, when the concentrations of CdSe/S²⁻ NCs and Cd²⁺ were increasedto 10 mg/mL and 50 mM, respectively, immediate precipitation of CdSe NCs(FIG. 51) was observed. At the same time, the addition of 0.15 M NaNO₃with ionic strength identical to the ionic strength of 0.05 M Cd(NO₃)₂did not cause precipitation of CdSe/S²⁻ NCs (FIG. 51), ruling out theincrease of ionic strength as the cause of colloidal instability.Without wishing to be bound by theory, it was concluded thatprecipitation of NCs was a result of linking CdSe/S²⁻ NCs with Cd²⁺ ionsas schematically shown in FIG. 39B. The elemental analysis ofprecipitated CdSe/S²⁻/Cd²⁺ NCs revealed no original K⁺ counter ions,suggesting that Cd²⁺ completely displaced K⁺ ions during aggregation ofNCs. This treatment was then applied to the close-packed films ofCdSe/S²⁻ NCs, originally charge-balanced with K⁺ ions. The absorptionspectra of CdSe/S²⁻ NC films red-shifted by ˜10 nm (38 meV) andbroadened after dipping into a 50 mM solution of Cd(NO₃)₂ in methanol,suggesting enhanced electronic coupling between the NCs (FIG. 39A). Togain a better insight to the electronic coupling strength, chargecarrier mobility was measured in the arrays of CdSe/S²⁻NCs bridged withdifferent cations. For these studies bottom-gated thin-film FETs werefabricated, with top source and drain electrodes (FIG. 39C). The films(˜20 nm thick) of K₂S capped CdSe NCs were deposited by spin coating onhighly doped Si wafers with 100 nm thick SiO₂ thermal gate oxide. The NCfilms were treated with different cations, by dipping into 50 mMsolutions of corresponding metal nitrates in methanol, followed byrinsing the films with neat solvent. The films were then annealed at200° C. followed by patterning source and drain Al electrodes by ashadow mask (channel length 150 μm, width 1500 μm). This annealing stepallowed evaporation of residual solvent molecules coordinated to NCsurface and further increased electronic coupling between CdSe NCs,evidenced by additional broadening of the excitonic peaks (FIG. 39A). Atthe same time, this annealing temperature was insufficient to sinterCdSe NCs, as seen by the blue shift of the absorption onset with respectto the bulk CdSe band gap (1.74 eV or 713 nm) and by powder X-raydiffraction pattern typical for 4.2 nm CdSe NCs (FIG. 52).

The FET devices assembled from CdSe/S²⁻ NCs showed n-type transport.FIGS. 40A and 40B show the output characteristics (i.e., drain currentI_(DS) as a function of source-drain voltage V_(DS) at different gatevoltages V_(G)) for FETs made of pristine films of CdSe/S²⁻NCs with K⁺counter ions and the same films cross-linked with Cd²⁺ ions. Thecross-linking of NCs with Cd²⁺ resulted in ˜100-fold increase in I_(DS)values for comparable V_(G) values. Large difference in the channelcurrents can be seen in the transfer curves (I_(DS) vs. V_(G) at fixedV_(DS)) for both devices (FIG. 40C). When positive bias is applied tothe gate terminal (V_(G)>0), electrons are injected into the FET channeland provide additional doping to the NC film resulting in a dramaticincrease of the channel current. The ratio of drain currents atV_(G)=30V and V_(G)=−10V was I_(ON)/I_(OFF)˜10⁴ and ˜10⁷, for thepristine and Cd²⁺ treated CdSe/S²⁻ NC films, respectively (FIG. 53). Theslope of the I_(DS) vs. V_(G) curves at V_(DS)=3V was used to calculatethe electron mobility corresponding to the linear regime of FEToperation (I_(lin)). It revealed a large increase in the electronmobility, from μ_(lin)=0.01 for NC films with K⁺ counter ions to 1.3cm²V⁻¹s⁻¹ for those bridged with Cd²⁺. Reproducible changes in electronmobility were observed for the films treated with other cations likeMn²⁺, Zn²⁺ and In³⁺, establishing a generic trend for NCs linked withdifferent cations (FIGS. 40G and 54). The large increase of carriermobility after Cd²⁺ or Zn²⁺ treatment was caused by the linking adjacentNCs, which also explain the broadening and red-shift in UV-visibleabsorption spectra (FIG. 39A). The extent of enhancement of electronmobility with different metal ions may depend on both the chemicalaffinity of the metal to NC surface, and the electronic structure of theNC/S²⁻-M^(n+)-S²⁻/NC interface. Experiments above showed that CdSe/ZnSand CdSe/CdS core/shell NCs showed lower electron mobility compared toCdSe NCs; particularly, CdSe/ZnS NCs exhibited rather poor electronmobility (˜10⁻⁵ cm²V⁻¹s⁻¹) because of additional electron tunnelingbarriers imposed by the shell material. In a similar manner, it isexpected that μ_(lin) in metal-bridged all-inorganic NC solids shouldcorrelate with the alignment of the NC 1S_(e) state and the LUMO of theS-M^(n+)-S bridge. Indeed, the comparison of the conduction bandenergies (E_(CB)) for corresponding metal sulfidesE_(CB)(K₂S)>E_(CB)(MnS)>E_(CB)(ZnS)>E_(CB)(In₂S₃)>E_(CB)(CdS) correlateswell with the observed mobility trend in FIG. 40G.

The effect of cationic metal ion cross-linking was also observed forIII-V InAs NCs (FIGS. 40D-40F, 40H and 55), again showing a clearenhancement of the electron mobility after bridging InAs/S²⁻ NCs withmetal ions. The cation linking phenomenon was also found effective forCdSe NCs capped with Li₂S and K₂Se (FIG. 56). The use of metalcross-links in combination with rapid thermal annealing allowed for theachievement of rather high electron mobilities in NC solids, up toμ_(lin)=7 cm²V⁻¹s⁻¹ (FIG. 57). Such high FET mobility was achieved usingNCs processed from an environmentally benign FA solvent.

Carrier mobility reflects the connectivity between neighboring NCswhereas the FET turn-on gate voltage (V_(th)) reflects the initialconcentration of charge carriers in the channel (i.e., the dopinglevel). Generally, lower V_(th) implies higher doping of the NC solid.The effect of different cations on V_(th) was compared and it wasobserved that CdSe/S²⁻ NCs bridged with In³⁺ ions showed a lower V_(th)than those bridged with Cd²⁺ ions (inset to FIG. 40C). In turn, the NCswith K⁺ counter ions showed even higher V_(th) values. Without wishingto be bound by theory, this trend can be explained by drawing acomparison between inter-NC bridges (FIG. 39B) and substitutionaldopands in conventional semiconductors. Indeed, when a monovalent cation(e.g., K⁺) occupies divalent (Cd²⁺) lattice site, it typically createsan acceptor level and behaves as a p-type dopant. On the other hand,In³⁺ integrated in CdSe lattice behaves as the n-type dopant. A similartrend in V_(th) was observed for InAs NCs bridged with K⁺, Cd²⁺ and In³⁺ions. Without wishing to be bound by theory, it can be concluded thatmetal ions bridging all-inorganic NCs strongly interact with NCs and cancontrol not only the carrier mobility but the concentration of chargecarriers in the NC solids.

Surface Metal Ions Determining the Type of Majority Carriers inall-Inorganic Nanocrystal Solids.

The ability of colloidal NC cationic metal ion cross-links to controldoping in NC solids was examined by studying charge transport in CdTe NCarrays. CdTe is a rather unusual II-VI semiconductor that can supportboth p-type and n-type transport, depending on the native defects andimpurity doping. K₂Te capped colloidal CdTe NCs were prepared followingstandard organic-to-inorganic ligand exchange. The films ofas-synthesized CdTe/Te²⁻/K⁺ NCs formed a p-type transistor channel(FIGS. 41A and 41E, and 58). Depending on the FET geometry and electrodematerial, measured hole mobility was in the range 10⁻⁶-10⁻⁴ cm²/Vs.P-type mobility was also dominant when the films of K₂Te capped CdTe NCswere treated with Cd²⁺ ions (FIG. 59). When CdTe/Te²⁻ NCs was treatedwith In³⁺ ions, obtained NC solids exhibited ambipolar transport (FIGS.41B, 41C, 41F, and 41G), with significantly enhanced n-type transportcompared to the NC films treated with Cd²⁺. To further enhance n-typetransport in the CdTe NC solids, the concentration of In³⁺ ions at theNC surface was increased by using In₂Se₄ ²⁻ metal chalcogenide complexesinstead of electron-accepting Te²⁻ ligands. CdTe/In₂Se₄ ²⁻ NC solidsshowed strong n-type conductivity with electron mobility approaching 0.1cm²/Vs (FIGS. 41C and 60).

Magnetic Properties of CdSe and InAs NCs Linked with Mn² Ions.

In addition to tuning the electronic properties of colloidal NC solids,cationic cross-links can be used to introduce new functionalities toall-inorganic NC solids. As an example, Mn²⁺ ions can be used tointroduce magnetic functionality to non-magnetic CdSe/S²⁻ NCs.

For example, a colloidal solution of K₂S capped CdSe NCs in FA wasprecipitated by adding Mn²⁺ ions. The elemental analysis showed acomplete displacement of K⁺ with Mn²⁺, leading to Mn-to-Cd ratio of1:6.7 corresponding to about 13 molar % Mn²⁺ with respect to Cd²⁺. Thisvalue sets an upper limit, corresponding to about 0.3 monolayerscoverage of 4.2 nm CdSe NC surface with Mn²⁺ ions. Lower doping level(e.g., 0.5% Mn) can be easily achieved by using mixtures of Mn²⁺ andCd²⁺ ions (1:5.7 molar ratio) for NC linkage. In all cases, the XRDpatterns of CdSe NCs remained identical before and after the Mn²⁺treatment suggesting binding Mn²⁺ to the NC surface without forming anyimpurity phases (FIG. 61).

Bulk CdSe is diamagnetic with temperature-independent magneticsusceptibility X_(m)˜−3.3·10⁻⁷ cm³/g. A diamagnetic response of 4.2 nmCdSe NCs capped with K₂S (X_(m)˜−7·10⁻⁷ cm³/g) was observed. FIG. 42shows magnetization (M) vs. magnetic field strength (H) plots forCdSe/S²⁻ NCs before and after exchange of K⁺ ions for Mn²⁺. Mn²⁺ ionsimparted positive magnetic susceptibility combined with a hystereticmagnetization with a small coercivity ˜40 Oe at 5 K, which vanished at300 K (FIG. 42, top left inset). At room temperature, CdSe NC solidsbridged with Mn²⁺ ions showed field-independent paramagnetic responsewith X_(m)˜5.9·10⁻⁶ cm³/g. Similar behavior was observed for 4.5 nmInAs/S²⁻ NCs linked with Mn²⁺ ions. In the case of InAs/S²⁻/Mn²⁺ NCs,high-temperature paramagnetic behavior can be described by theCurie-Weiss law 1/χ=(T−θ)/C where C is the Curie constant, T istemperature, and e is the Curie temperature (FIG. 62A). The negative θvalue −22 K is indicative of the antiferromagnetic (AFM) exchangeinteraction between Mn²⁺ ions. Similar AFM coupling was observed forMn²⁺ doped into the lattice of various semiconductors both in bulk andNCs. The magnetic susceptibility of CdSe/S²⁻/Mn²⁺ NCs showed morecomplex behavior, with a significant contribution fromtemperature-independent paramagnetism (FIGS. 62B, 62C).

The bottom right inset in FIG. 42 shows zero-field-cooled (ZFC) andfield-cooled (FC, 100 Oe) magnetization data for CdSe/S²⁻/Mn²⁺ NCs,revealing a magnetic transition with the blocking temperature about 34K, that can be similar to the formation of a ferrimagnetic phase in CdSeNCs doped with Mn²⁺ ions. These magnetic studies show that bridgingall-inorganic semiconductor NCs with paramagnetic Mn²⁺ ions can lead tobehaviors typical for dilute magnetic semiconductor NCs. When combinedwith the electronic properties provided by inorganic ligands, furtherapplications are possible for capped NCs, such as spintronicapplications. For example, charging of NCs by applying gate voltage,could induce ferromagnetic ordering of the magnetic moments of Mn²⁺d-electrons. One may also expect fundamental differences between CdSeNCs substitutionally doped with Mn²⁺ and those bridged with Mn²⁺ ions.This comes from different local environment for Mn²⁺ ions, possibledifferences in the oxidation state and the interaction of metal bridgeswith the NC core. The local environment is of great importance forcolloidal NC cationic metal ion cross-linkages. Applied electronparamagnetic resonance (EPR), X-ray Absorption Near Edge Structure(XANES), and Extended X-Ray Absorption Fine Structure (EXAFS) techniqueswere used to better understand the bonding and local structure Mn²⁺ ionsbetween all-inorganic NCs.

Probing the Local Environment of Mn² Ions Linking CdSe NCs by EPR, XANESand EXAFS Spectroscopy.

Mn²⁺ has five unpaired electrons and its local environment can be probedby EPR spectroscopy. FIG. 43 shows EPR spectrum measured at 4.6 K forCdSe/S²⁻ NCs cross-linked with Mn²⁺ ions. This spectrum can bedeconvoluted into two overlapping components: (i) a broad signal arisingfrom the exchange coupled Mn²⁺ ions, and (ii) a characteristic sextetarising from the hyperfine interaction of the electron spin with Mnnucleus (I=5/2) in isolated Mn²⁺ ions. Measured Landé g-factor 2.002 iscloser to that for Mn²⁺:CdS than to the g-factor for Mn²⁺:CdSe (2.004),which suggests that the local coordination of Mn²⁺ ions can besulfur-rich, in accord with the bonding shown in FIG. 37A. At the sametime, the hyperfine coupling constant A_(iso)=86 10⁻⁴ cm⁻¹ (9.2 mT), issignificantly larger than that for tetrahedrally coordinated Mn²⁺ inbulk CdS and CdSe lattices (65.10⁻⁴ cm⁻¹ and 62.10⁴ cm⁻¹, respectively),The large value of hyperfine splitting is characteristic of Mn²⁺ ionsresiding at the NC surface. For example, A_(iso)=85×10⁻⁴ cm⁻¹ wasreported for Mn²⁺ ions localized at the surface of CdSe NCs, but forMn²⁺ doped into a II-VI semiconductor NC lattice, A_(iso)<70.10⁻⁴ cm⁻¹.

4.5 nm InAs NCs capped with S²⁻ ligands and bridged with Mn²⁺ ionsrevealed similar g-factor and A_(iso) (FIG. 63), suggesting that both inCdSe/S²⁻/Mn²⁺ and InAs/S²⁻/Mn²⁺ NCs, the manganese ions are bound to thesurface through the same ligand, and are not part of the crystalstructure of the substrate. At the same time, upon illuminating thesamples of CdSe/S²⁻/Mn²⁺ and InAs/S²⁻/Mn²⁺ NCs with a 300 W Xe-lampthrough an IR-blocking filter and a long-pass filter with cutoff at 400nm, the EPR intensity decreased by 7%. The variation of the EPRintensity was fully reversible with the light on/off cycles shown in theinset to FIG. 43. The use of illumination above 400 nm (hv<3.1 eV) ruledout the direct excitation of MnS clusters at the NC surface becauseE_(g)(MnS)˜3.7 eV. The intensity of the EPR signal remained independentof illumination when a long-pass filter with a cutoff at 700 nm wasused, which was smaller than the bang gap of CdSe NCs (FIG. 64). Thecontrol experiments also ruled out the variation in sample temperaturecaused by illumination. The effective concentration of Mn²⁺ decreasesunder light, and recovered completely upon shutting light off suggestingan efficient coupling between Mn²⁺ ions in CdSe/S²⁻/Mn²⁺ NCs andlight-absorbing CdSe NCs. Such coupling could occur either via energytransfer exciting Mn²⁺ from ⁶A₁ to ⁴T₁ state or via the charge transferto Mn²⁺ ions reducing its total spin. The decrease in EPR intensity mayalso arise from the antiferromagnetic p-d coupling of thevalence-band-hole with Mn²⁺, which dominates over the ferromagnetic s-sinteraction of conduction-band-electron with Mn²⁺, particularly for Mn²⁺ion residing near the NC surface. Further studies will be necessary toclarify the spin dynamics in all-inorganic NCs.

The XANES spectroscopy provided additional information about theoxidation of surface Mn and S atoms at the surface of all-inorganic NCswhile the EXAFS spectroscopy was employed to probe the local environmentaround Mn ions on the surface of CdSe/S²⁻ NCs. The degree of edge shift,obtained via comparison of Mn K-edge XANES for the Mn²⁺ treated NCsample (CdSe/S²⁻/Mn²⁺) with different Mn standards (FIG. 44A) isindicative of a +2 oxidation state, similar to that in bulk MnS. At thesame time, differences in the shape of the XANES spectra forCdSe/S²⁻/Mn²⁺ NCs and MnS indicate the local structure around Mn²⁺ inthe NC sample is unique. Particularly, in the NC sample, the Mn K-edgeXANES revealed a strong pre-edge absorption peak (highlighted with thearrow in FIGS. 44A and 65) that corresponds to the dipole-forbidden1s→3d transition. The intensity of this transition allows probing thelocal environment for Mn²⁺. Thus, the pre-edge peak is absent or veryweak for centrosymmetric metal sites with O_(h) symmetry typical for MnSand MnO phases with rock salt structure. The observation of a strong1s→3d transition suggests non-centrosymmetric, e.g., tetrahedral (T_(d))coordination of Mn²⁺ ions. The relative intensities of the pre-edge(1s→3d) and edge (1s→4p) transition in CdSe/S²⁻/Mn²⁺ NCs were similar tothose reported for isoelectronic Fe³⁺ sites with T_(d) symmetry inFe-doped Na₂SiO₃ glass and for tetrahedral Mn²⁺ doped into ZnO wurtzitelattice.

Further information on the coordination environment for Mn²⁺ sites inCdSe/S²⁻ /Mn²⁺ NCs can be obtained from the analysis of EXAFS spectra.FIG. 44B shows the magnitude of the Fourier transform (FT) of Mn K-edgeEXAFS data for CdSe/S²⁻/Mn²⁺ NC sample and the corresponding fit usingMn—S path generated from MnS with zinc blende structure using FEFF6. Thefitting procedure together with the real part of the FT data are shownin FIG. 66 and Table 2. This analysis suggests that Mn atoms are bondedto S atoms with average Mn—S coordination number (CN)=3.1, and bondlength=2.37 Å. The obtained CN is slightly smaller than the typicaltetrahedral (CN=4, wurtzite and zinc blend) and octahedral (CN=6, rocksalt) coordination. At the same time, obtained bond length is close tothe theoretical bond length of Mn—S path in the MnS with wurtzite andzinc blende phases (˜2.41 Å) while significantly shorter than the bondlength 2.62 Å for Mn—S path in MnS with rock salt structure. Thisreduced Mn—S CN and bond length for CdSe/S²⁻/Mn²⁺ NC sample suggest Mn²⁺binding exclusively to the NCs surface, which was also demonstratedindependently by XRD and EPR studies. The combination of intensepre-edge peak observed in XANES spectra with the bond length and CNobtained from EXAFS fits rule out the possibility of the formation ofMnS phase and support T_(d) symmetry of Mn²⁺ sites at the NC surface.

TABLE 2 List of the fit parameters for Mn K-edge fit of Mn²⁺ treated K₂Scapped CdSe (CdSe/S²⁻/Mn²⁺) NCs. Data range: k is from 2.6-8 Å⁻¹ and thefit range: r is from 1.0-3.0 Å. S_(o) ² = 1 Bond Path Coordinationnumber length (Å) σ² (Å²) ΔE (eV) Mn—S 3.1 ± 0.5 2.37 ± 0.02 0.01 ±0.003 −6.0 ± 1.6 (σ² = mean square displacement of the distance betweenthe atoms, ΔE = energy shift). R-factor for the fit is 0.02.

The S-edge XANES data of K₂S capped CdSe NCs before (CdSe/S²⁻/K⁺) andafter (CdSe/S²⁻/Mn²⁺) Mn²⁺ treatment along with several sulfur standardsare shown in FIGS. 44C and 67. The difference of XANES spectra betweenK₂S and CdSe/S²⁻/K⁺ NCs agrees with the preconceived notion that S²⁻binds covalently to the Cd²⁺ sites at the surface of CdSe NCs. Moreimportantly, the XANES data for CdSe/S²⁻/Mn²⁺ NC samples can bereasonably well fitted as a superposition of S-edge XANES spectra forCdSe/S²⁻/K⁺ NCs and MnS. A linear combination fit shows that the XANESspectrum for CdSe/S²⁻/Mn²⁺ NCs can be modeled as 55% contribution fromCdSe/S²⁻/K⁺ NC and 45% from MnS; once again suggesting the formation ofCd—S—Mn bonds on the NC surface (FIG. 68 and Table 3).

TABLE 3 S-edge XANES Linear Combination Fit Result: Sample S-stdPercentage R-factor CdSe—Mn—S CdSe—S 0.55 .003 MnS 0.45 CdSe—S—K₂S CdS0.85 .006 K₂S 0.15

Combined together, EPR, XANES and EXAFS studies suggest that Mn²⁺ ionsbind to the surface of CdSe/S²⁻ NCs without changes of their oxidationstate. The Mn²⁺ primarily bind to the electron-rich S²⁻ ligands at theNC surface with tetrahedral-like coordination environment, averagecoordination number 3.1 and Mn—S bond length 2.37 Å. At the same time,S²⁻ ligands bind simultaneously to Cd²⁺ and Mn²⁺ ions, behaving asbridges for electronic coupling for paramagnetic Mn²⁺ ions to CdSe NCs.

Pt²⁺ Bridges can be Used to Enhance the Electrocatalytic Properties ofall-Inorganic Nanocrystal Solids.

Engineered nanomaterials formed of colloid particles represent animportant class of prospective photo- and electrocatalysts that combinetunable electronic structure with high surface-to-volume ratio. Tooptimize the catalytic efficiency, the NC surface can be modified withoxidation or reduction catalysts, including, but not limited to, noblemetal nanoparticles, MoS₃ or even enzymes. The present techniques showthat transition metal ions bound to the surface of all-inorganic NCs canbe used to mediate redox processes at the NC surface. To demonstratethis effect, the electrochemical properties of CdSe/S²⁻ nanocrystals wascompared with different counter ions, K⁺ and Pt²⁺, in a modelelectrochemical reaction of proton reduction and hydrogen evolution inacetonitrile and aqueous media (FIG. 45A). The first set ofelectrochemical measurements was performed in anhydrous acetonitrileinside air-free glovebox. Protonated DMF treflate [H(DMF)]OTf, behavingas a weak acid with pK_(a)=6.1 in acetonitrile, was used as a protonsource and tetrabutylammonium perchlorate (TBAP) was used as anelectrolyte. The CdSe/S²⁻/K⁺ NC films were deposited on eitherITO-covered glass or on glassy carbon (GC) electrodes from formamidesolutions. As-deposited films showed the first absorption peak near 590nm. Treatment of CdSe/S²⁻/K⁺ NC films with 0.1 M formamide solution of[PtCl₄]²⁻ for 30 minutes resulted in a 5 nm red shift of firstabsorption peak (FIG. 69) indicating binding of Pt²⁺ to NC surface,which probably enhanced the electronic coupling between NCs analogous tothe data shown in FIG. 39A. In control experiments, it was confirmedthat neither the shape of the absorption spectrum nor the position ofthe first excitonic peak changed after five cycles of the electrodepotential between −1.7 V and 0 V vs. Ag⁺/Ag reference electrode at 100mV/s (E_(Ag+/Ag)=−0.125V relative to Cp₂Fe⁺/Cp₂Fe couple), indicatingthat CdSe NCs stayed intact in the course of electrochemical studies(FIG. 69).

Cyclic voltammetry of CdSe/S²⁻/K⁺ and CdSe/S²⁻/Pt²⁺ NC films on a GCelectrode in presence of 0.01 M [H(DMF)]OTf showed diffusion limitedproton reduction. The wave for reduction of protons on GC electrode wasobserved with a half-wave potential of −0.778 V vs Ag⁺/Ag referenceelectrode (FIG. 70). It corresponds to the overpotential of ˜403 mV.Depositing CdSe/S²⁻/K⁺ NC film on a GC surface increased theoverpotential to ˜410 mV (FIG. 45B, black curve). The exchange of K⁺ions with Pt²⁺ resulted in significantly lower overpotential of ˜273 mV(FIG. 45B, blue curve), in agreement with increased catalytic activityof the NC surface. To study electrocatalytic performance ofall-inorganic NCs is aqueous medium, similar measurements were carriedout in aqueous solutions buffered at pH 6.5 with a phosphate buffer. Atthis pH, the Nernst equation gives E(H₂/2H⁺)=−0.38 V relative to the SHEor −0.58 V relative to the saturated Ag/AgCl/Cl⁻ reference electrodeused in this work. However, no hydrogen reduction was observed up to−1.4 V on glassy carbon (GC) because of low exchange currents for thisreaction leading for high overpotentials. The deposition of 4.2 nmCdSe/S²⁻/K⁺ NC film on a GC electrode resulted in a minor improvement ofcatalytic activity (FIG. 45C, black curve). After replacing the K⁺ ionswith Pt²⁺ by exposing the NC film to 0.1 M FA solution of K₂[PtCl₄] for30 minutes followed by careful rinse with acetonitrile, a strongenhancement of the faradaic current was observed, with the current onsetshifted to −0.736 V vs Ag/AgCl/Cl (or −0.539 V vs SHE) as shown in FIG.45C. The Tafel analysis revealed ˜50-fold enhancement of the exchangecurrent, from ˜6 nA/cm² to 0.3 μA/cm² when K⁺ ions were replaced withPt²⁺. In the control experiments, polished GC electrode or a film ofTOPO-capped CdSe NCs was exposed to 0.1M FA solution of K₂[PtCl₄] andmeasured the effect of such treatment on electrochemical properties. Inboth cases, only minor changes in cathodic current were observed beforeand after treatment with K₂[PtCl₄] (FIG. 71). These results can beexplained by inefficient adsorption of Pt²⁺ on GC and TOPO-cappedsurface of CdSe NCs. In contrast, the exchange of K⁺ ions with Pt²⁺ inCdSe/S²⁻/K⁺ NC solids occurred very efficiently at room temperaturebecause of accessibility of NC surface and the presence of negativelycharged S²⁻ groups exhibiting high chemical affinity to Pt(II) species.This also demonstrates that charge carriers in all-inorganicsemiconductor NCs can efficiently interact with redox catalysts at theNC surface.

Thus as shown, cationic metal ions can interact with the surface ofcolloidal particles, such as all-inorganic semiconductor NCs, and allowengineering the NC properties both in solution and in the NC solids. Forexample, Cd²⁺, Ca²⁺, Zn²⁺ can efficiently bind to S²⁻capped CdSe orCdSe/ZnS core shell NCs in dilute solution inversing the surface chargefrom negative to positive, and significantly increasing the luminescenceefficiency. The cationic metal ions can bridge neighboring NCs andcontrol the magnetic and electrochemical properties of all-inorganicNCs. In an array of K₂S capped CdSe NCs the treatment with suitablemetal ions allows replacing weakly bound K⁺ ions and linking adjacentNCs. The formation of cross-linked NC structures enhances charge carriermobility by two orders of magnitude, leading to the electron mobility1.3 cm²V⁻¹s⁻¹ achieved via a benign hydrazine-free device processing.The cationic metal ion cross-links also allow tailoring the doping inthe NC solids, for example, switching CdTe NC FETs from p-type toambipolar to n-type. EPR and EXAFS spectroscopy carried out for CdSe NCswith S²⁻ ligands bridged by Mn²⁺ ions confirmed that Mn²⁺ bonded to theS²⁻ capped CdSe NC via S—Mn—S bonds with bond length 2.37 Å and averagecoordination number 3.1. NCs with Mn²⁺ on surface exhibitsuperparamagnetic behavior. Magnetization studies showed that bridgingwith Mn²⁺ ions can switch the magnetic properties of NC solids fromdiamagnetic to paramagnetic at room temperature and to a magneticallyordered phase at low temperature. Finally, it was demonstrated that thebridging CdSe NCs with Pt²⁺ ions allows improving the electrocatalyticproperties of all-inorganic NCs in the hydrogen evolution reaction, bothin aqueous and acetonitrile media. These examples show the generalityand versatility of the approach described herein for tailoring physicaland chemical properties for a broad class of NC-based materials.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, it will be apparent to those of ordinaryskill in the art that changes, additions and/or deletions may be made tothe disclosed embodiments without departing from the spirit and scope ofthe invention.

What is claimed:
 1. A colloidal material comprising nanoparticles andinorganic capping agents, wherein each inorganic capping agent is boundto an outer surface of a nanoparticle, and the inorganic capping agentscomprise H_(n)MO_(y), where n=0, 1, or 2, y=2, 3, or 4, M is a metal,metalloid or phosphorus and H_(n)MO_(y) is negatively charged.
 2. Thecolloidal material of claim 1, wherein M is P, As, W, V, or Mo.
 3. Thecolloidal material of claim 1, wherein the inorganic capping agentscomprise VO₄ ³⁻, MoO₄ ²⁻, WO₄ ²⁻, PO₄ ³⁻, AsO₄ ³⁻, HPO₃ ²⁻, H₂PO₂ ⁻, ora mixture thereof.
 4. A colloidal material comprising nanoparticles andinorganic capping agents, wherein the inorganic capping agents are boundto at least a portion of the surface of the nanoparticles and theinorganic capping agents comprise H₃[PMo₁₂O₄₀]; H₃[PW₁₂O₄₀];Na₃PMo₁₂O₄₀; Na₆H₂W₁₂O₄₀; H₄[SiW₁₂O₄₀]; (NH₄)₆[H₂W₁₂O₄₀]; K₆[P₂W₁₈O₆₂];K₆[P₂Mo₁₈O₆₂]; Mo₁₅₄; Rb₈K₂[{Ru₄O₄(OH)₂; (H₂O)₄}(γ-SiW₁₀O₃₆)₂], or amixture or derivative thereof.
 5. The colloidal material of claim 1,wherein the colloidal material is a super-lattice.
 6. The colloidalmaterial of claim 1 made by a method comprising admixing inorganiccapping agents in a first solvent and nanoparticles-in a second solventtogether to form the colloidal material, wherein the second solvent isappreciably immiscible in the first solvent.
 7. A method of making thecolloidal material of claim 1, comprising (a) admixing the nanoparticlesin a first solvent and the inorganic capping agents in a second solventin the presence of a quaternary ammonium salt to form the colloidalmaterial; and (b) isolating the colloidal material from the admixture,wherein the first solvent is nonpolar and the second solvent is polar.8. A matrix comprising a plurality of colloidal particles, eachcomprising a nanoparticle and an inorganic capping agent, wherein theinorganic capping agent is bound to at least a portion of thenanoparticle surface, wherein the inorganic capping agent comprisesH_(n)MO_(y), where n=0, 1, or 2, y=2, 3, or 4, M is a metal, metalloidor phosphorus and H_(n)MO_(y) is negatively charged, and wherein pairsof colloidal particles are bridged by a cationic ion cross-linkagesbound to the respective colloidal particles of the pair.
 9. A fieldeffect transistor comprising: a source region and a drain region and thematrix of claim 8 extending between, and electrically coupled to, thesource region and the drain region to provide current flow between thesource region and the drain region, in response to activation of thefield effect transistor by a gate coupled to the matrix and having athreshold gate voltage.
 10. The colloidal material of claim 4, whereinthe colloidal material is a super-lattice.
 11. The colloidal material ofclaim 4 made by a method comprising admixing inorganic capping agents ina first solvent and nanoparticles in a second solvent together to formthe colloidal material, wherein the second solvent is appreciablyimmiscible in the first solvent.
 12. A method of making the colloidalmaterial of claim 4, comprising (a) admixing the nanoparticles in afirst solvent and the inorganic capping agents in a second solvent inthe presence of a quaternary ammonium salt to form the colloidalmaterial; and (b) isolating the colloidal material from the admixture,wherein the first solvent is nonpolar and the second solvent is polar.13. A matrix comprising a plurality of colloidal particles, eachcomprising a nanoparticle and an inorganic capping agent, wherein theinorganic capping agent is bound to at least a portion of thenanoparticle surface, wherein the inorganic capping agent comprisesH₃[PMo₁₂O₄₀]; H₃[PW₁₂O₄₀]; Na₃PMo₁₂O₄₀; Na₆H₂W₁₂O₄₀; H₄[SiW₁₂O₄₀];(NH₄)₆[H₂W₁₂O₄₀]; K₆[P₂W₁₅O₆₂]; K₆[P₂Mo₁₈O₆₂]; Mo₁₅₄; Rb₅K₂[{Ru₄O₄(OH)₂;(H₂O)₄}(γ-SiW₁₀O₃₆)₂], or a mixture or derivative thereof, and whereinpairs of colloidal particles are bridged by a cationic ioncross-linkages bound to the respective colloidal particles of the pair.14. A field effect transistor comprising: a source region and a drainregion and the matrix of claim 13 extending between, and electricallycoupled to, the source region and the drain region to provide currentflow between the source region and the drain region, in response toactivation of the field effect transistor by a gate coupled to thematrix and having a threshold gate voltage.